fix(#1995): widen worktree branch regex to accept agent-<id> namespace (#2548)

* test(#1995): regression test for agent-<id> branch namespace

Add failing-first tests proving that normalizeCleanupManifestEntry and
planWorktreeRecordAgent reject Claude Code's current agent-<id> isolation
branches (only worktree-agent-<id> is accepted). Boundary tests cover both
namespaces plus rejection cases.

* fix(#1995): widen worktree branch regex to accept agent-<id> namespace

Claude Code's isolation="worktree" branch naming changed from
worktree-agent-<id> to agent-<id>. Widen the regex in all 7 locations
from ^worktree-agent-[A-Za-z0-9._/-]+$ to ^(worktree-)?agent-[A-Za-z0-9._/-]+$
so both namespaces are accepted. Introduce a shared WORKTREE_AGENT_BRANCH_RE
constant in src/worktree-safety.cts to prevent future drift.

Closes #1995

* fix(#1995): update workflow guards, test assertions, and baselines

Widen the branch-check regex in execute-phase.md and execute-plan.md.
Update all test assertions that checked for ^worktree-agent- to expect
the widened ^(worktree-)?agent- pattern. Regenerate golden-install-parity
fixtures, agent-size-baseline, and workflow-size-baseline.

Closes #1995

* fix(#1995): update extractCwdGuardBash sanity check for widened regex

The e2e test's sanity check verified the extracted bash block contained
'worktree-agent-'. After widening to '(worktree-)?agent-', update the
check to match the new pattern.

* fix(#1995): widen missed workflow-guard branch check + changeset + lint fixes

- hooks/gsd-workflow-guard.js: widen startsWith('worktree-agent-') to
  /^(worktree-)?agent-/ regex — same defect class, was missed in prior commit
- tests/worktree.test.cjs: fix indentation regression from prior edit
- Add .changeset/1995-worktree-agent-branch-namespace.md (pr:0 placeholder)

Found by orthogonal code review (Step 4).

* fix(#1995): regenerate golden + size baselines for workflow-guard change

* docs(#1995): backfill changeset PR number (2548)

agents/gsd-executor.md

@@ -495,11 +495,11 @@ if [ -f .git ]; then  # worktree
     echo "DO NOT use 'git update-ref' to rewind the protected branch — surface as blocker (#2924)." >&2
     exit 1
   fi
-  # Positive allow-list: HEAD must be on the canonical Claude Code worktree-agent
-  # branch namespace (`worktree-agent-<id>`). This catches feature/* and any other
-  # arbitrary branch that the deny-list would silently allow (#2924).
-  if ! echo "$ACTUAL_BRANCH" | grep -Eq '^worktree-agent-[A-Za-z0-9._/-]+$'; then
-    echo "FATAL: refusing to commit — worktree HEAD '$ACTUAL_BRANCH' is not in the worktree-agent-* namespace." >&2
+  # Positive allow-list: HEAD must be on a per-agent branch (`agent-<id>` or
+  # legacy `worktree-agent-<id>`). This catches feature/* and any other
+  # arbitrary branch that the deny-list would silently allow (#2924, #1995).
+  if ! echo "$ACTUAL_BRANCH" | grep -Eq '^(worktree-)?agent-[A-Za-z0-9._/-]+$'; then
+    echo "FATAL: refusing to commit — worktree HEAD '$ACTUAL_BRANCH' is not in the agent-* / worktree-agent-* namespace." >&2
     echo "Agent commits must live on per-agent branches; surface as blocker (#2924)." >&2
     exit 1
   fi