feat(mcp): scan-stage tool — Go port of the stage-scan validator

The corezoid-stage-scan skill shipped a 270-line Python validator as a
user-machine runtime dependency outside the Go-only CI. It is now the
scan-stage MCP tool inside convctl (single-runtime plugin): same five
defect classes ([1] inactive, [1b] empty shells, [2a] broken node
links, [2b] broken/inactive conv refs, [2c] api_get_task node missing
in the target), same folder-location annotations, quiet and json
modes; a parity run on a fixture covering every class produced
identical findings to the Python original. Non-object JSON files are
skipped silently, exactly like Python's isinstance guard.

Deliberate divergences, all safety-positive: zip extraction refuses
zip-slip entries and re-roots absolute names (Python's extractall had
no guard); the export path is confined to the working directory like
every other file tool here; files with duplicate or missing obj_id
are each scanned and listed (the Python dict silently dropped
duplicates and crashed on several missing-obj_id shapes). The CLI
exit code reflects tool errors only — the skill's CI recipe gates on
the 'No blockers found.' verdict line instead.

The skill drives the tool; the Python script is deleted and the
checks run as ordinary go tests.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: ivkou

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- Feat: `scan-stage` MCP tool — the corezoid-stage-scan validator ported from the skill's Python script to Go inside convctl (single-runtime plugin, zip-slip-safe extraction, deterministic output; `quiet` and `json` modes). The skill now drives the tool; `scripts/scan_stage.py` is gone and the checks are covered by go tests.
 - Feat: code-enforced node placement on `push-process` — new nodes added with placeholder coordinates (`x: 0, y: 0`) are auto-placed by the MCP server. Preserve mode is the default: already-placed nodes are never moved, only the new `(0,0)` nodes are slotted near their graph neighbours without overlap; a fully-new process gets a clean layered layout. Disable with the environment variable `COREZOID_AUTOLAYOUT=off`.
 - Feat: `layout-process` MCP tool — the deterministic auto-layout engine (waterfall for simple trees, sugiyama-lite + error rail for meshes, aligned TABLE/STAR region grids), fully in Go inside convctl. It rewrites only `x`/`y` and the `extra.modeForm` collapse flag, preserves the source file's indentation and trailing newline, and always reports the chosen strategy, canvas size and overlap count; `dry=true` previews without writing, `density=compact|medium|roomy` controls spacing. The corezoid-node-layout skill now drives this tool — its Python scripts are gone (single-runtime plugin), with the 12 layout invariants running as ordinary go tests plus golden coordinate files. Also fixes the hybrid strategy dropping `modeForm=collapse` on residual-graph nodes (they rendered as full blocks shifted by the collapsed offset).
 - Feature: new `corezoid-node-layout` skill — auto-arranges a process's node x/y into a clean, readable layout and rewrites the `.conv.json` in place (positions + IF/Delay/error collapse only; edges, logic, `conv_id`, aliases and node types are left byte-for-byte intact). Business logic is laid out as a vertical top-to-bottom spine; error handling (escalation → reply → error-final, reachable only via `err_node_id`) is collapsed into a tidy right-hand rail; unreachable orphans go into a compact grid. Guarantees no node overlaps and coordinates within Corezoid's ±10000 canvas (the vertical step shrinks for very deep processes). Simple tree-like processes use a "waterfall" (branches fanned around a central column); large mesh processes use a layered algorithm (dummy nodes for long edges + median crossing-minimisation + priority coordinate straightening). The engine is implemented in Go inside the plugin's MCP server (the `layout-process` tool above — single-runtime plugin, no Python); its clean-layout invariants run as ordinary go tests. Complements the existing `docs/process/node-positioning-best-practices.md`. Run it as the last step before `push-process`.

README.md

@@ -213,6 +213,7 @@ validation errors, and summarize what each process does.
 | `push-process`      | Validate and deploy a `.conv.json` to Corezoid     |
 | `layout-process`    | Auto-arrange node coordinates (waterfall / layered / table-star regions); local, changes only x/y and collapse flags |
 | `lint-process`      | Validate process structure locally (no API call)   |
+| `scan-stage`        | Static validation of an exported stage .zip/dir: inactive processes, broken node links and conv refs (local, no API) |
 | `run-task`          | Send a task to a deployed process                  |
 | `list-node-tasks`   | List tasks currently sitting in a node             |
 | `list-task-history` | Show task execution history                        |

plugins/corezoid/mcp-server/mcp_handlers.go

@@ -31,6 +31,7 @@ var toolHandlers = map[string]toolHandler{
 	"delete-variable":      handleDeleteVariable,
 	"push-process":         handlePushProcess,
 	"layout-process":       handleLayoutProcess,
+	"scan-stage":           handleScanStage,
 	"lint-process":         handleLintProcess,
 	"run-task":             handleRunTask,
 	"create-process":       handleCreateProcess,
@@ -102,6 +103,7 @@ var toolHandlers = map[string]toolHandler{
 // occurred before or during the login flow.
 var noAuthTools = map[string]struct{}{
 	"layout-process": {},
+	"scan-stage":     {},
 	"lint-process":   {},
 	"login":          {},
 	"logout":         {},