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)

tests/fixtures/golden-install-parity/opencode.json

@@ -16,7 +16,7 @@
   "agents/gsd-domain-researcher.md": "71250e759ca9e723",
   "agents/gsd-eval-auditor.md": "c88890105f32ace6",
   "agents/gsd-eval-planner.md": "60bddb70a937f796",
-  "agents/gsd-executor.md": "4946be74860ea68e",
+  "agents/gsd-executor.md": "14423bfb4dee14b8",
   "agents/gsd-framework-selector.md": "1c0a10355e787675",
   "agents/gsd-integration-checker.md": "a9de5928e5a5c649",
   "agents/gsd-intel-updater.md": "493e07482fa6198a",
@@ -232,7 +232,7 @@
   "gsd-core/references/verification-patterns.md": "3bd98d30eee6c5a1",
   "gsd-core/references/verify-mvp-mode.md": "534bdc7f2432903a",
   "gsd-core/references/workstream-flag.md": "225910640adbb76d",
-  "gsd-core/references/worktree-branch-check.md": "21d9c31bf6542b93",
+  "gsd-core/references/worktree-branch-check.md": "161547f45f78c2bd",
   "gsd-core/references/worktree-path-safety.md": "3c8d74756f9b16a8",
   "gsd-core/templates/AI-SPEC.md": "24df5fe5ba34e367",
   "gsd-core/templates/DEBUG.md": "72e93b7f650d9bae",
@@ -318,13 +318,13 @@
   "gsd-core/workflows/docs-update.md": "f6210dc8ff7776d5",
   "gsd-core/workflows/edit-phase.md": "1876c855fb0a0a39",
   "gsd-core/workflows/eval-review.md": "1973f08d0eb34159",
-  "gsd-core/workflows/execute-phase.md": "4e19bf9ead5fbafc",
+  "gsd-core/workflows/execute-phase.md": "b59b9993b1da7bb8",
   "gsd-core/workflows/execute-phase/steps/codebase-drift-gate.md": "a15993affd62f4bf",
   "gsd-core/workflows/execute-phase/steps/per-plan-worktree-gate.md": "7ebb7d1af6082028",
   "gsd-core/workflows/execute-phase/steps/post-merge-gate.md": "c4cba01539f0368a",
   "gsd-core/workflows/execute-phase/steps/regression-gate.md": "21f4d9b3abc334ce",
   "gsd-core/workflows/execute-phase/steps/worktree-recovery-policy.md": "be84efbd71e1513e",
-  "gsd-core/workflows/execute-plan.md": "a2e611fff6af9c29",
+  "gsd-core/workflows/execute-plan.md": "bb0ce725dcf12aa2",
   "gsd-core/workflows/explore.md": "a514b88e98609441",
   "gsd-core/workflows/extract-learnings.md": "92b3c0979604b7d0",
   "gsd-core/workflows/fast.md": "12187b242e6af970",
@@ -418,8 +418,8 @@
   "hooks/gsd-validate-commit.sh": "bf5dd61d33cb3a38",
   "hooks/gsd-windsurf-pre-command.js": "948be1c6d14c79cd",
   "hooks/gsd-windsurf-pre-write.js": "92d4dbfbc36ab0cf",
-  "hooks/gsd-workflow-guard.js": "d0cea21bc596fd27",
-  "hooks/gsd-worktree-path-guard.js": "c8f6c980305ae6fd",
+  "hooks/gsd-workflow-guard.js": "56414206bc13b3dc",
+  "hooks/gsd-worktree-path-guard.js": "bfef1802d491bdab",
   "hooks/lib/git-cmd.js": "268ba15992ca0b23",
   "hooks/lib/gsd-graphify-rebuild.sh": "66af89601074d2a9",
   "hooks/managed-hooks-registry.cjs": "bd57cc72f482a14f",