fix(ci,engine): repair test sharding, case-variant ambiguity detection, post-merge CI

Test shards 3 and 4 were silently failing on every open PR because vitest's
CLI parser was treating `--shard X/Y` as positional file filters whenever the
arg arrived after a `--` separator. Removing the `--` in ci-test-shard.mjs
restores per-shard slicing; verified locally that shard 1/4 and 2/4 now run
distinct subsets.

The two consistently-failing engine tests:

1. self-healing in-review-branch-rebind ambiguous case-variant detection:
   dedup keyed on lowercase branch name collapsed two physically distinct
   refs (allowed on Linux ext4) into one candidate, so the "applied" path
   ran instead of "ambiguous-candidates". Dedup now keys on the resolved
   SHA — macOS APFS still collapses (same ref, same SHA), Linux keeps both
   (distinct SHAs) and the ambiguity skip path fires as designed.

2. worktree-acquisition resume-misbinding spy: the production
   verifyResumeBranchNotMisbound returns early when `git merge-base HEAD main`
   fails, which is exactly what happens on shallow checkouts. Bumping the
   test-shards checkout to fetch-depth: 0 makes CI mirror the local git
   state these engine tests rely on.

Also adds `push: branches: [main]` to PR Checks so regressions like this
(which slipped into v0.33.0 with no post-merge run) go red immediately
on landing instead of being discovered on the next PR.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/pr-checks.yml

@@ -3,6 +3,10 @@ name: PR Checks
 on:
   pull_request:
     branches: [main]
+  # Also run on every push to main so post-merge regressions surface
+  # immediately instead of being discovered on the next PR.
+  push:
+    branches: [main]
 
 concurrency:
   group: pr-checks-${{ github.ref }}
@@ -69,6 +73,12 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          # Engine tests run real git operations (merge-base against main,
+          # case-variant ref checks) that require full history. Shallow
+          # clones silently break tests like worktree-acquisition's resume
+          # misbinding path.
+          fetch-depth: 0
 
       - name: Setup Node.js and pnpm
         uses: ./.github/actions/setup-node-pnpm