fix(#3503): derive the code-review diff base from GSD's own commit scopes, not prose mentions (#3526)

* fix(#3503): derive the code-review diff base from GSD's own commit scopes, not prose mentions

The #2989/#3191 anchor ('[Pp]hase N' + POSIX boundary) still resolved the
phase diff base ~4 phases early on real repos: git log --grep searches full
commit bodies and tail -1 keeps the OLDEST match, so a single prose mention
anywhere in history (a planning commit forward-referencing the phase per
D-09, a doc commit using '### Phase N' as a format example) silently
captured the base — while GSD's own commits, which use conventional-commit
scopes (docs(phase-6):, feat(6-01):, docs(06):) and never contain the
literal 'Phase N', were matched by nothing. The wrong base inflated the
Tier-3 file-list fallback, the #2666 SUMMARY/diff union, the reviewer
agent's diff_base, and fallow's --changed-since scope, with no warning.

All three derivation sites (Tier-3 fallback, spawn_reviewer, fallow
structural pre-pass) now grep for the subject-line conventional-commit
phase scope under --extended-regexp, in lockstep per the #3191 contract:

  ^[[:alpha:]]+!?\((phase-)?(N|0N)(-[0-9]+)?\)!?:

PHASE_SCOPE_NUM accepts both padded and unpadded phase spellings because
workflows emit the unpadded roadmap number (docs(phase-6):) while
code-review greps the zero-padded PADDED_PHASE. The ^ anchor makes it a
subject-line match, so commit-body prose can never capture the base. The
POSIX-ERE portability rule (#3191, no \b), the fail-closed empty-result
warning, and the --files escape hatch are preserved: histories with no
scope-style commits yield no base instead of an arbitrary one.

Tests (tests/code-review-pipeline-regression.test.cjs): new Bug 6 (#3503)
block executes the SHIPPED bash from all three sites against a real git
fixture whose history carries every prose false-positive class from the
issue — red pre-fix (the prose-body commits captured the base at every
site), green post-fix. The Bug 5 (#3191) block is updated to the scope
anchor contract (its fixtures bound to the shipped text), and its T6
docs-parity guard now enforces the identical scope-anchored grep plus the
PHASE_SCOPE_NUM prep at every git-log site.

Emitted drift: 3503-diff-base-scope-anchor.json acks the deliberate
workflow growth; the spent 3191-unanchored-grep-sites.json fragment (its
code-review.md entry was consumed when #3191 merged) is pruned.

* chore(#3503): add changeset fragment for PR #3526

* fix(#3503): rebase onto next and correct the emitted-drift ack

Rebase onto origin/next@6badb839 (PR freshness: #3514/#3516 landed after
this branch was cut). Post-rebase the attribution gate classifies the two
source-path ack entries (gsd-core/workflows/code-review.md, structural-
pre-pass.md) as stale — source files present in the diff are identity-
attributed by the table, so only the emitted code-review.md basename
growth needs an acknowledgment. The fragment now names exactly that one
consumed entry.

---------

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

gsd-core/workflows/code-review.md

@@ -237,14 +237,26 @@ against the diff and warn about (then add) any changed files the SUMMARY extract
 surface — so a partial SUMMARY result can no longer silently mask the rest of the phase.
 ```bash
 # Compute diff base from phase commits — fail closed if no reliable base found.
-# #2989: anchor the grep to the phase-mention convention ("Phase N" / "phase N")
-# so a bare digit substring doesn't match version strings, dates, issue refs,
-# or other phases' numbers. When no commit genuinely references the phase, this
-# yields empty and the fail-closed warning below actually fires.
-# #3191: the trailing boundary is the POSIX class ([^[:alnum:]_]|$), NOT \b —
-# \b is not a POSIX ERE token, so under --extended-regexp it silently matches
-# nothing on macOS regex(3), making this fallback dead on Apple platforms.
-PHASE_COMMITS=$(git log --oneline --all --grep="[Pp]hase ${PADDED_PHASE}([^[:alnum:]_]|$)" --extended-regexp --format="%H" 2>/dev/null)
+# #3503: anchor the grep to GSD's own conventional-commit phase scopes — the
+# subject-line formats this system itself emits (docs(phase-N): from
+# execute-phase.md, plan scopes feat(N-MM):/test(N-MM): from references/tdd.md,
+# bare phase scopes docs(N):). The #2989/#3191 prose anchor "[Pp]hase N"
+# matched free prose in ANY commit body — planning commits forward-reference
+# later phases ("deferred to Phase N per D-09"), doc commits use "### Phase N"
+# as a format example — and tail -1 (oldest match) turned each false positive
+# into a base unboundedly before the phase, while GSD's own scope commits
+# never contain the literal "Phase N" at all. The ^ anchor makes this a
+# subject-line match, so commit-body prose can never capture the base.
+# Workflows emit the UNPADDED roadmap phase number (docs(phase-6):) while
+# PADDED_PHASE is zero-padded ("06") — accept both spellings.
+# #3191: stay POSIX-ERE portable — the boundary is the closing paren + colon,
+# never \b (not a POSIX ERE token; under --extended-regexp it silently matches
+# nothing on macOS regex(3), making this fallback dead on Apple platforms).
+PHASE_SCOPE_NUM="${PADDED_PHASE}"
+case "${PADDED_PHASE}" in
+  0[0-9]*) PHASE_SCOPE_NUM="${PADDED_PHASE#0}|${PADDED_PHASE}" ;;
+esac
+PHASE_COMMITS=$(git log --oneline --all --extended-regexp --grep="^[[:alpha:]]+!?\((phase-)?(${PHASE_SCOPE_NUM})(-[0-9]+)?\)!?:" --format="%H" 2>/dev/null)
 DIFF_BASE=""
 if [ -n "$PHASE_COMMITS" ]; then
   DIFF_BASE=$(echo "$PHASE_COMMITS" | tail -1)^
@@ -425,13 +437,17 @@ Compute the review output path:
 REVIEW_PATH="${PHASE_DIR}/${PADDED_PHASE}-REVIEW.md"
 ```
 
-Compute DIFF_BASE for agent context (in case agent needs it). #3191: this must be
-the SAME anchored, POSIX-portable derivation the Tier-3 scope step uses — the
-reviewer agent consumes `diff_base` exactly when `files:` is empty, i.e. the same
-fail-closed scenario Tier 3 protects, so a divergent unanchored recomputation here
-re-arms the mis-scoping one tier down:
+Compute DIFF_BASE for agent context (in case agent needs it). #3191/#3503: this
+must be the SAME anchored, POSIX-portable conventional-commit-scope derivation
+the Tier-3 scope step uses — the reviewer agent consumes `diff_base` exactly
+when `files:` is empty, i.e. the same fail-closed scenario Tier 3 protects, so
+a divergent recomputation here re-arms the mis-scoping one tier down:
 ```bash
-PHASE_COMMITS=$(git log --oneline --all --grep="[Pp]hase ${PADDED_PHASE}([^[:alnum:]_]|$)" --extended-regexp --format="%H" 2>/dev/null)
+PHASE_SCOPE_NUM="${PADDED_PHASE}"
+case "${PADDED_PHASE}" in
+  0[0-9]*) PHASE_SCOPE_NUM="${PADDED_PHASE#0}|${PADDED_PHASE}" ;;
+esac
+PHASE_COMMITS=$(git log --oneline --all --extended-regexp --grep="^[[:alpha:]]+!?\((phase-)?(${PHASE_SCOPE_NUM})(-[0-9]+)?\)!?:" --format="%H" 2>/dev/null)
 if [ -n "$PHASE_COMMITS" ]; then
   DIFF_BASE=$(echo "$PHASE_COMMITS" | tail -1)^
   # Verify the parent commit exists (first commit in repo has no parent)