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)

gsd-core/workflows/execute-phase.md

@@ -445,7 +445,7 @@ cwd inside an agent worktree (or a subdirectory of one). Every subsequent
 orchestrator-side git call would then target the wrong tree — this is how a wrong-base
 merge nearly shipped ~1000 files. Resolve the *worktree root* (so a subdirectory cwd
 cannot skew the check) and refuse if it is an agent worktree. The discriminator is the
-per-agent branch namespace `worktree-agent-*`, NOT the `.claude/worktrees/` path: the
+per-agent branch namespace `agent-*` / `worktree-agent-*`, NOT the `.claude/worktrees/` path: the
 orchestrator may itself be legitimately invoked from a feature worktree under
 `.claude/worktrees/`, so a path-substring refusal would break legitimate runs. Do NOT
 pin to `git worktree list`'s first entry — that is the main worktree, the wrong target
@@ -455,7 +455,7 @@ when the orchestrator legitimately runs from a feature worktree.
 ORCHESTRATOR_WT=$(git rev-parse --show-toplevel 2>/dev/null) || {
   echo "FATAL: execute_waves entry is not inside a git worktree (#48)." >&2; exit 1; }
 ORCH_BRANCH=$(git rev-parse --abbrev-ref HEAD 2>/dev/null)
-if printf '%s' "$ORCH_BRANCH" | grep -Eq '^worktree-agent-'; then
+if printf '%s' "$ORCH_BRANCH" | grep -Eq '^(worktree-)?agent-'; then
   echo "FATAL: orchestrator cwd is inside an agent worktree (branch '$ORCH_BRANCH', root '$ORCHESTRATOR_WT') — refusing to execute waves (#48). A prior isolation=\"worktree\" dispatch drifted the cwd; re-run from the orchestrator's own worktree." >&2
   exit 1
 fi