File Suggestions For `@` Mentions In Chat Input

Summary:

Add file-path suggestions to the chat input when the active token matches `@` followed by at least one non-whitespace character. Show suggestions below the input, allow `↑`/`↓` navigation, and on `Tab` replace the active `@query` token with the selected file path plus one trailing space. Use `ripgrep` first to gather project files, with a Node.js recursive fallback similar to `grepSearch()`.

Key Changes:

- Update `src/components/Chat/Input.tsx` to:
  - keep owning the input string and reset/remount behavior for `TextInput`
  - detect file suggestion mode from the active token using `(^|\s)@\S+`
  - render `FileSuggestions` only when slash-command mode is not active
  - apply the callback result from `FileSuggestions` by replacing the input value and remounting `TextInput`
- Add `src/components/Chat/FileSuggestions.tsx` to own:
  - extraction of the active `@query` from the provided input value
  - project file discovery
  - filtering and focused-index navigation
  - `Tab` confirmation
  - rendering the suggestion list below the input with at most `5` visible options
- Keep slash command behavior unchanged:
  - if input starts with `/`, continue rendering `CommandMenu`
  - file suggestions do not appear while slash-command mode is active
  - `Enter` continues to submit the input normally; it does not confirm file suggestions
- Implement file discovery inside `FileSuggestions.tsx` using:
  - first choice: `rg --files` from `process.cwd()`
  - fallback: recursive `readdirSync(..., { withFileTypes: true })`
  - normalization to relative paths with `/` separators
  - include hidden files/directories in suggestions
  - exclude only `.git` from the fallback traversal
- Keep matching simple and case-insensitive against the path text after `@`.
- Have `FileSuggestions` call `onSelect(nextInput: string)` so it returns the fully rewritten input string, including the trailing space, instead of only the raw file path.
- Do not support `Esc` dismissal for file suggestions; the list should open and close solely based on the current input text and normal editing.

Public Interfaces / Behavior:

- No CLI/API changes.
- New TUI behavior:
  - typing `@` followed by any non-whitespace character in the active token opens file suggestions below the input
  - `↑`/`↓` move focus through suggestions
  - `Tab` inserts the focused file path into the input and appends one space
  - `Enter` still submits the current input and does not confirm the suggestion list
  - suggestions disappear automatically when the active token no longer matches `@\S+` or when no files match

Test Plan:

- Add `Input` tests for:
  - no file suggestions for plain text or bare `@`
  - file suggestions appear for `@s`, `@1`, `@_`, or `@foo-bar`
  - slash command suggestions still win for leading `/`
  - `Tab` inserts the focused file path and appends one trailing space
  - insertion replaces only the active trailing `@query` token and preserves surrounding text
  - `Enter` still submits while file suggestions are visible
  - `Backspace` updates or closes file suggestions correctly
  - disabled input ignores file suggestion interactions
- Add `FileSuggestions` tests for:
  - `rg --files` success path
  - fallback to Node.js traversal when `rg` fails
  - fallback excludes `.git` but includes other hidden paths
  - case-insensitive filtering
  - focused-index movement and `Tab` selection behavior
  - max visible option count is `5`

Assumptions:

- Suggestion source is project file paths relative to the current working directory.
- `ripgrep` is optional; the feature must work without it via the Node.js fallback.
- `FileSuggestions` will contain both its own logic and rendering; no separate helper module is introduced initially.
- The input remains synchronized by remounting `TextInput` with a refreshed `defaultValue` after a selection is inserted.