fix(mcp): pin gittensory_propose_action to the PR current head

[JSONbored]

What

proposeAction built the staged action's params from only label/reviewBody/mergeMethod/closeComment — it never fetched or recorded the PR's head SHA. So every MCP-staged action had expectedHeadSha === undefined, and the approval-queue accept path's force-push guard (stagedHead && stagedHead !== pr.headSha) is a silent no-op on a falsy stagedHead: a maintainer or agent could propose a merge, the contributor could force-push unreviewed code, and accept would merge the new commit with no supersede/rejection — exactly the threat that guard exists to catch, just never engaged for this entrypoint.

Fix

Fetch the PR's current stored head via getPullRequest before building params and pin expectedHeadSha to it, matching the invariant the live webhook/sweep staging path (stageForApproval) already upholds. Uses the stored/DB-synced value (not a fresh live GitHub fetch), consistent with how the existing staging path resolves this — no added GitHub API load on a maintainer-facing MCP call.

Tests

• expectedHeadSha is now staged when a PR record exists (and correctly omitted when it doesn't — matches an existing test's baseline).
• End-to-end: propose a merge, force-push the PR, accept — the existing force-push guard now actually fires (status: "rejected", executionOutcome: "head_moved"), where before this fix it silently proceeded to execute.

Full unsharded test:coverage green; typecheck green.

Advances #1936. Closes #2255.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.71%. Comparing base (d02fbb9) to head (b1515f7).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2355   +/-   ##
=======================================
  Coverage   95.71%   95.71%           
=======================================
  Files         222      222           
  Lines       24653    24655    +2     
  Branches     8945     8946    +1     
=======================================
+ Hits        23597    23599    +2     
  Misses        433      433           
  Partials      623      623           

Files with missing lines	Coverage Δ
src/mcp/server.ts	95.72% <100.00%> (+0.01%)	⬆️

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