feat(diff): add layout = "inline" for VS Code-style unified diff (#195)

## Summary

Add a new `layout = "inline"` option for `diff_opts` that renders a VS
Code-style unified inline diff - a single read-only buffer with deleted
(red/strikethrough) and added (green) lines interleaved. This
complements the existing `"vertical"` and `"horizontal"` two-panel diff
layouts.

Closes #82 

## Motivation

The existing diff layouts use Neovim's built-in `diffthis` to show old
and new content side by side. While effective, this uses significant
screen real estate with two panels. An inline unified diff provides a
more compact view that's familiar to VS
  Code users, showing changes in context within a single buffer.

## Changes

- **New module** `lua/claudecode/diff_inline.lua` — self-contained
inline diff implementation with:
    - Pure diff computation via `vim.diff()` (indices mode)
    - Extmark-based rendering with sign column markers (`+`/`-`)
    - Resolution (accept/reject) and cleanup functions
- **Dispatch in `diff.lua`** — routes to inline module when `layout =
"inline"` is configured; exposes shared utilities as `M._` members
following existing patterns
  - **Config/types** — validates `"inline"` as a valid layout value
- **`close_all_diff_tabs`** — detects inline diff buffers via
`claudecode_inline_diff` buffer variable
- **Test mock** — adds `vim.diff` mock with LCS-based hunk computation
to `tests/mocks/vim.lua`
- **23 new tests** covering diff computation, content extraction,
rendering, MCP response format, config validation, and cleanup

## Configuration

  ```lua
  require("claudecode").setup({
    diff_opts = {
layout = "inline", -- "vertical" (default), "horizontal", or "inline"
    },
  })
```

Highlight groups are customizable:
  - ClaudeCodeInlineDiffAdd — green background for added lines
  - ClaudeCodeInlineDiffDelete — red background + strikethrough for deleted lines
  - ClaudeCodeInlineDiffAddSign / ClaudeCodeInlineDiffDeleteSign — sign column colors

  Requirements

  - layout = "inline" requires Neovim >= 0.9.0 (for vim.diff())
  - The plugin's base requirement remains Neovim >= 0.8.0; only inline layout needs 0.9+

  Design Decisions

  1. Separate module rather than inlining into diff.lua — keeps the already-large file manageable; inline diff uses fundamentally different rendering (extmarks vs diffthis)
  2. layout field in diff state — enables dispatch in resolution/cleanup without re-reading config
  3. Read-only buffer — inline diff is review-only; content extraction uses saved line_types array rather than parsing buffer text
  4. Always vsplit — single buffer doesn't need horizontal/vertical choice

  Test Plan

- [x] luacheck lua/ tests/ passes with 0 warnings / 0 errors
- [x]  All 406 tests pass (23 new, 5 pre-existing failures unrelated to this change)
- [x]  Manual: layout = "inline" → single-window diff with colored lines
- [x]  Manual: Accept (<leader>aa) → file saved correctly, diff closes
- [x]  Manual: Deny (<leader>ad) → file unchanged, diff closes
- [x]  Manual: New file creation → all lines shown as green/added
- [x]  Manual: layout = "vertical" still works unchanged (regression check)

---------

Signed-off-by: Thomas Kosiewski <tk@coder.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Thomas Kosiewski <tk@coder.com>

Author: 3 people

README.md

@@ -371,7 +371,11 @@ For deep technical details, see [ARCHITECTURE.md](./ARCHITECTURE.md).
 
     -- Diff Integration
     diff_opts = {
-      layout = "vertical", -- "vertical" or "horizontal"
+      layout = "vertical", -- "vertical" (default), "horizontal", or "inline"
+      -- "inline": VS Code-style unified diff in a single buffer with deleted
+      --   (red/strikethrough) and added (green) lines interleaved. Requires
+      --   Neovim >= 0.9.0. Highlight groups are customizable: ClaudeCodeInlineDiffAdd,
+      --   ClaudeCodeInlineDiffDelete, ClaudeCodeInlineDiffAddSign, ClaudeCodeInlineDiffDeleteSign.
       open_in_new_tab = false,
       keep_terminal_focus = false, -- If true, moves focus back to terminal after diff opens
       hide_terminal_in_new_tab = false,

lua/claudecode/config.lua

@@ -130,8 +130,10 @@ function M.validate(config)
   -- New diff options (optional validation to allow backward compatibility)
   if config.diff_opts.layout ~= nil then
     assert(
-      config.diff_opts.layout == "vertical" or config.diff_opts.layout == "horizontal",
-      "diff_opts.layout must be 'vertical' or 'horizontal'"
+      config.diff_opts.layout == "vertical"
+        or config.diff_opts.layout == "horizontal"
+        or config.diff_opts.layout == "inline",
+      "diff_opts.layout must be 'vertical', 'horizontal', or 'inline'"
     )
   end
   if config.diff_opts.open_in_new_tab ~= nil then

lua/claudecode/diff.lua

@@ -826,6 +826,13 @@ function M._resolve_diff_as_saved(tab_name, buffer_id)
     return
   end
 
+  -- Dispatch to inline diff handler
+  if diff_data.layout == "inline" then
+    local inline = require("claudecode.diff_inline")
+    inline.resolve_inline_as_saved(tab_name, diff_data)
+    return
+  end
+
   logger.debug("diff", "Accepting diff for", tab_name)
 
   -- Get content from buffer
@@ -913,6 +920,13 @@ function M._resolve_diff_as_rejected(tab_name)
     return
   end
 
+  -- Dispatch to inline diff handler
+  if diff_data.layout == "inline" then
+    local inline = require("claudecode.diff_inline")
+    inline.resolve_inline_as_rejected(tab_name, diff_data)
+    return
+  end
+
   -- Create MCP-compliant response
   local result = {
     content = {
@@ -1135,6 +1149,14 @@ function M._cleanup_diff_state(tab_name, reason)
     return
   end
 
+  -- Dispatch to inline diff handler
+  if diff_data.layout == "inline" then
+    local inline = require("claudecode.diff_inline")
+    inline.cleanup_inline_diff(tab_name, diff_data)
+    active_diffs[tab_name] = nil
+    return
+  end
+
   -- Clean up autocmds
   for _, autocmd_id in ipairs(diff_data.autocmd_ids or {}) do
     pcall(vim.api.nvim_del_autocmd, autocmd_id)
@@ -1276,6 +1298,13 @@ function M._setup_blocking_diff(params, resolution_callback)
       end
     end
 
+    -- Dispatch to inline diff if configured
+    if config and config.diff_opts and config.diff_opts.layout == "inline" then
+      local inline = require("claudecode.diff_inline")
+      inline.setup_inline_diff(params, resolution_callback, config)
+      return
+    end
+
     local original_tab_number = vim.api.nvim_get_current_tabpage()
     local created_new_tab = false
     local terminal_win_in_new_tab = nil
@@ -1742,6 +1771,18 @@ end
 ---This function reads the diff context from buffer variables
 function M.accept_current_diff()
   local current_buffer = vim.api.nvim_get_current_buf()
+
+  -- Check for inline diff buffer first
+  if vim.b[current_buffer].claudecode_inline_diff then
+    local tab_name = vim.b[current_buffer].claudecode_diff_tab_name
+    if tab_name then
+      M._resolve_diff_as_saved(tab_name, current_buffer)
+    else
+      vim.notify("No active diff found in current buffer", vim.log.levels.WARN)
+    end
+    return
+  end
+
   local tab_name = vim.b[current_buffer].claudecode_diff_tab_name
 
   if not tab_name then
@@ -1756,6 +1797,18 @@ end
 ---This function reads the diff context from buffer variables
 function M.deny_current_diff()
   local current_buffer = vim.api.nvim_get_current_buf()
+
+  -- Check for inline diff buffer first
+  if vim.b[current_buffer].claudecode_inline_diff then
+    local tab_name = vim.b[current_buffer].claudecode_diff_tab_name
+    if tab_name then
+      M._resolve_diff_as_rejected(tab_name)
+    else
+      vim.notify("No active diff found in current buffer", vim.log.levels.WARN)
+    end
+    return
+  end
+
   local tab_name = vim.b[current_buffer].claudecode_diff_tab_name
 
   if not tab_name then
@@ -1767,6 +1820,14 @@ function M.deny_current_diff()
   M._resolve_diff_as_rejected(tab_name)
 end
 
+-- Expose internal utilities for use by diff_inline.lua
+M._find_main_editor_window = find_main_editor_window
+M._find_claudecode_terminal_window = find_claudecode_terminal_window
+M._is_buffer_dirty = is_buffer_dirty
+M._detect_filetype = detect_filetype
+M._get_autocmd_group = get_autocmd_group
+M._display_terminal_in_new_tab = display_terminal_in_new_tab
+
 return M
 ---@alias NvimWin integer
 ---@alias NvimBuf integer