fix(#3191): anchor remaining diff-base greps, portably (#3437)

* fix(#3191): anchor remaining diff-base greps, portably

The #2989 fix anchored only the Tier-3 grep, and did so with \b — not a
POSIX ERE token, so on macOS regex(3) it silently matches nothing and
Tier 3 always fails closed. spawn_reviewer's agent-context DIFF_BASE and
the fallow structural pre-pass's --changed-since base each still ran the
original unanchored --grep="${PADDED_PHASE}", whose oldest substring
match is routinely a version-string/date commit from months before the
phase existed — feeding the reviewer agent a bogus diff_base exactly
when files: is empty, and widening fallow's changed-files scope.

All three derivations now use the same anchored, POSIX-portable
'[Pp]hase N([^[:alnum:]_]|$)' with --extended-regexp; spawn_reviewer
also gains Tier-3's parent-exists guard so the two computations are the
same algorithm. Behavioral regression tests execute the shipped bash
extracted from the workflow files against a git fixture on every
platform, so the macOS \b hole is covered, not just the Linux CI view.

* chore(#3191): backfill changeset PR number 3437

* fix(#3191): scope fallow test snippet past the gsd-tools resolver

The CI runners have no installed gsd-tools, so executing the resolver
line that precedes FALLOW_SCOPE_ARGS in the extracted fence exits 1
before the derivation under test ever runs. Slice the snippet to start
at FALLOW_SCOPE_ARGS=() — the resolver is orthogonal to the base
derivation the regression test binds.

---------

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

.changeset/curious-tunas-fly.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 3437
+---
+code-review: every phase diff-base derivation now uses the same anchored, POSIX-portable phase-mention grep. Fixes wrong review scope from /gsd:code-review when a phase has no SUMMARY artifacts: the reviewer diff_base and the fallow --changed-since base no longer resolve to old unrelated commits whose messages merely contain the phase digits, and the anchored search now actually matches on macOS (the previous \b word boundary is not POSIX ERE and silently matched nothing there).