Detect wrong MCP binary in integration-test skill (#154)

[obj-p]

Fixes #154.

Summary

When iterating in a git worktree nested under the main repo (e.g., .claude/worktrees/<name>/), Claude Code launches the stdio MCP server from the parent repo's .build/, not the worktree's. /resume makes it sticky — it preserves the original project root regardless of relaunch CWD. The integration-test skill's preview_build_info staleness check (#147 / #150) happily reports green in this state: the binary at the resolved path does match its running process; it just isn't the binary the developer is iterating on.

Hit live during PR #153 validation — a full integration-test run reported green against a parent-repo binary that didn't have any of the PR's changes.

Approach

Step 2 of the skill now runs two independent checks:

• 2a. Wrong-binary check. Derive expected path from git rev-parse --show-toplevel of the skill's CWD, resolve symlinks the same way preview_build_info does, compare to its returned binaryPath. Mismatch aborts with a tailored relaunch message that explicitly names /resume as the trap.
• 2b. Stale-binary check. Existing logic, unchanged.

Both checks must pass before proceeding. They cover independent failure modes — wrong binary entirely vs stale binary at the right path.

Why not change preview_build_info itself

The MCP server has no way to know what the developer's current worktree is. Users who installed PreviewsMCP via brew / mint / a binary copy have no <root>/.build/<arch>/debug/previewsmcp layout to compare against — a server-side check would falsely flag every end-user install as "wrong tree." The skill is the only programmatic caller of preview_build_info and the right place for developer-mode context (CWD, expected build layout).

Documentation

Adds a "worktree footgun" paragraph to AGENTS.md alongside the existing "Refresh stdio" guidance, with a pointer to the skill's automatic detection.

Out of scope

The upstream Claude Code behavior — ${workspaceFolder} not expanding in .mcp.json command (anthropics/claude-code#3239, #9427), and /resume preserving the original project root regardless of relaunch CWD. Tracked separately upstream.

Test plan

• Skill change is markdown-only — verified the rendered Step 2 reads cleanly
• AGENTS.md addition reads cleanly in context
• Manual: simulated the abort message wording — it correctly names /resume as the cause
• Next time /integration-test runs in a worktree, Step 2 will exercise the new check end-to-end

🤖 Generated with Claude Code