fix(#3995): derive the review diff base from the phase directory (#4181)

* test(#3995): diff base keys on the phase directory, not commit subjects

All three derivation sites (Tier 3, spawn_reviewer, fallow pre-pass)
must anchor on the phase directory's first commit; the milestone-blind
repro (an archived milestone's same-numbered phase commit capturing
the base) is the failing-first row. #3191/#3503 rows reworked to the
directory-anchor contract; T6 docs-parity forbids any remaining
phase-scope message-grep site.

* fix(#3995): derive the review diff base from the phase directory

A phase number is unique within a milestone, not a repository; the
message grep had no milestone bound and tail -1 deliberately selected
the oldest same-numbered subject, dragging archived milestones phases
into the scope (7 files to 3388 plus the >50 depth downgrade). All
three lockstep sites now anchor on the first commit that added anything
under the phase own directory — the same anchor class
git-base-branch phaseStartCommit uses. ShellCheck baseline gains the
escaped fragment shifted parse signature.

Emitted-Drift-Ack-Growth: code-review.md — phase-directory anchor replaces the message-grep derivation at both sites (#3995)

* chore(#3995): backfill changeset pr number

---------

Co-authored-by: sim <sim@local>

scripts/lint-workflow-shellcheck-baseline.json

@@ -229,6 +229,11 @@
     "code": "1073",
     "message": "Couldn't parse this command expansion. Fix to allow more checks."
   },
+  {
+    "file": "gsd-core/workflows/code-review/steps/structural-pre-pass.md",
+    "code": "1083",
+    "message": "This { is literal. Check expression (missing ;/\\n?) or quote it."
+  },
   {
     "file": "gsd-core/workflows/code-review/steps/structural-pre-pass.md",
     "code": "2086",