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)

docs/CLI-TOOLS.md

@@ -612,7 +612,7 @@ node gsd-tools.cjs worktree record-agent \
   --manifest <path> --agent-id <id> --path <worktree> --branch <branch> --base <sha>
 ```
 
-**`worktree record-agent`** appends one `{agent_id, worktree_path, branch, expected_base}` entry to an already-initialized manifest, validating every field **at write time using the same rules the `cleanup-wave` reader enforces** — `--branch` must match the disposable `^worktree-agent-[A-Za-z0-9._/-]+$` namespace, and `--path`/`--branch`/`--base` must be non-empty. `--agent-id` is required (write-strict), even though the reader treats it as optional. A missing or garbled field — or a duplicate `(worktree_path, branch)` the reader would dedup away — fails loudly with a recovery hint and a non-zero exit **without** writing, instead of appending an under-populated or silently-dropped entry. Whitespace-only `--path`/`--base` are rejected (values are trimmed). The on-disk manifest shape is unchanged (the reader re-derives `allowed_bases`); the orchestrator still initializes the empty `{orchestrator_root, worktrees: []}` shell inline before any agent is recorded.
+**`worktree record-agent`** appends one `{agent_id, worktree_path, branch, expected_base}` entry to an already-initialized manifest, validating every field **at write time using the same rules the `cleanup-wave` reader enforces** — `--branch` must match the disposable `^(worktree-)?agent-[A-Za-z0-9._/-]+$` namespace (accepts both `agent-<id>` and legacy `worktree-agent-<id>`), and `--path`/`--branch`/`--base` must be non-empty. `--agent-id` is required (write-strict), even though the reader treats it as optional. A missing or garbled field — or a duplicate `(worktree_path, branch)` the reader would dedup away — fails loudly with a recovery hint and a non-zero exit **without** writing, instead of appending an under-populated or silently-dropped entry. Whitespace-only `--path`/`--base` are rejected (values are trimmed). The on-disk manifest shape is unchanged (the reader re-derives `allowed_bases`); the orchestrator still initializes the empty `{orchestrator_root, worktrees: []}` shell inline before any agent is recorded.
 
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