fix(merger): address code-review findings on autostash + observer

P0 — parsePorcelainZ rename/copy handling
  Git's -z porcelain emits `R  <new>\0<old>\0` for renames (and
  C for copies). The naive split-and-slice treated <old> as an
  independent dirty path, which made runObservedDestructiveSyncOp
  warn about phantom "cleared paths" whenever a rename was in
  flight. Now we detect R/C status and skip the trailing entry.

P1 — race-rescue loop unstages between attempts
  `git stash create` snapshots the index without clearing it, so
  iteration 2's `git add -A` would re-stage atop iteration 1's
  leftovers. Tree differences inside the loop then reflected stale
  staging rather than genuine new writes. Added a `git reset` at
  the top of each iteration so every attempt starts from a clean
  index baseline.

P1 — writeActiveMergerStatus is now atomic
  Switched from in-place writeFileSync to temp-file + renameSync.
  POSIX guarantees rename atomicity on the same filesystem, so a
  reader can no longer catch the file mid-flush and return a
  false-negative "no merger active" advisory.

P2 — Step regex em-dash clarity
  `[—\-:]` is functionally fine but obscures intent; switched to
  `(?:—|-|:)` so the em-dash branch is obvious. Added a test case
  for the em-dash separator.

New tests:
- parse-porcelain-z.test.ts (8 cases including renames + copies)
- em-dash case added to derive-subject-summary.test.ts

247/247 merger-suite tests pass.

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

.changeset/merger-review-fixes.md

@@ -0,0 +1,10 @@
+---
+"@runfusion/fusion": patch
+---
+
+Address code-review findings on the merger autostash work:
+
+- `parsePorcelainZ` now correctly handles rename/copy entries (`R` / `C` status), which emit two NUL-separated entries for one logical change. Previously the old name was treated as an independent dirty path, causing `runObservedDestructiveSyncOp` to emit spurious "cleared N path(s)" warnings whenever a rename was in flight.
+- The race-rescue loop in `stashUnrelatedRootDirChanges` now runs `git reset` between attempts so each `git add -A` starts from a clean index, preventing iteration-2+ stashes from drifting due to stale staging rather than genuine new writes.
+- `writeActiveMergerStatus` now writes the advisory file via temp-path + atomic `renameSync` so dashboard readers can't observe a partial write.
+- `deriveDeterministicSubjectSummary`'s Step regex switched from `[—\-:]` to `(?:—|-|:)` — same matches, but the em-dash intent is obvious to anyone auditing.