fix(#3819): widen executor's pre-commit guard beyond worktree mode (#4343)

* fix(#3819): widen executor's pre-commit guard beyond worktree mode

The pre-commit protected-branch assertion in the executor agent (#2924)
only fired inside a Claude Code worktree and matched a hardcoded
five-name branch list. It never ran in an ordinary checkout and never
covered this repo's own default branch ("next"), so gsd-executor could
commit planning-repo documents directly onto a shared checkout's
default branch with no PR ever created.

Widen the guard to run in every isolation mode, and resolve the
protected branch via the repository's actual default branch (with the
existing five-name list retained as a fallback when the resolver
itself cannot be invoked) plus any configured git.protected_branches.
Add a git.allow_default_branch_commits escape hatch for projects that
intentionally execute on their default branch. Also point the
separate <final_commit> commit helper back at the same guard, so it
cannot be sidestepped by that path.

Emitted-Drift-Ack-Growth: gsd-executor.md — widened pre-commit protected-branch guard (#3819); tightened comments to stay under the size cap.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* docs(#3819): backfill changeset PR number

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

---------

Co-authored-by: sim <sim@local>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

agents/gsd-executor.md

@@ -479,19 +479,30 @@ Prefer **relative paths** for all Edit/Write operations inside a worktree. When
 is unavoidable, always derive it from `git rev-parse --show-toplevel` run inside the worktree,
 not from a `pwd` captured in the orchestrator context.
 
-**0. Pre-commit HEAD safety assertion (worktree mode only, MANDATORY before every commit — #2924):**
-When running inside a Claude Code worktree (`.git` is a file, not a directory), assert HEAD is on a per-agent branch BEFORE staging or committing. If HEAD has drifted onto a protected ref, HALT — never self-recover via `git update-ref refs/heads/<protected>`:
+**0. Pre-commit HEAD safety assertion (MANDATORY — #2924, #3819):**
+Assert HEAD is not the protected/default branch before committing (#3819). If drifted onto it, HALT — never self-recover via `git update-ref refs/heads/<protected>`:
 ```bash
-if [ -f .git ]; then  # worktree
-  HEAD_REF=$(git symbolic-ref --quiet HEAD || echo "DETACHED")
-  ACTUAL_BRANCH=$(git rev-parse --abbrev-ref HEAD)
-  # Deny-list: never commit on a protected ref.
-  if [ "$HEAD_REF" = "DETACHED" ] || \
-     echo "$ACTUAL_BRANCH" | grep -Eq '^(main|master|develop|trunk|release/.*)$'; then
-    echo "FATAL: refusing to commit — worktree HEAD is on '$ACTUAL_BRANCH' (expected per-agent branch)." >&2
-    echo "DO NOT use 'git update-ref' to rewind the protected branch — surface as blocker (#2924)." >&2
-    exit 1
+HEAD_REF=$(git symbolic-ref --quiet HEAD || echo "DETACHED")
+ACTUAL_BRANCH=$(git rev-parse --abbrev-ref HEAD)
+if [ "$HEAD_REF" = "DETACHED" ]; then
+  echo "FATAL: refusing to commit — HEAD is detached." >&2
+  exit 1
+fi
+# #3819: real default branch; override git.allow_default_branch_commits; else five-name fallback.
+IS_PROTECTED=$(gsd_run query git.base-branch --is-protected "$ACTUAL_BRANCH" 2>/dev/null) || IS_PROTECTED="__GSD_RUN_UNAVAILABLE__"
+if [ "$IS_PROTECTED" = "__GSD_RUN_UNAVAILABLE__" ] || [ -z "$IS_PROTECTED" ]; then
+  if echo "$ACTUAL_BRANCH" | grep -Eq '^(main|master|develop|trunk|release/.*)$'; then
+    IS_PROTECTED="true"
+  else
+    IS_PROTECTED="false"
   fi
+fi
+if [ "$IS_PROTECTED" != "false" ]; then
+  echo "FATAL: refusing to commit — HEAD is on '$ACTUAL_BRANCH' (protected/default branch)." >&2
+  echo "Re-home onto a phase/agent branch (#2924, #3819); override: git.allow_default_branch_commits:true in .planning/config.json." >&2
+  exit 1
+fi
+if [ -f .git ]; then  # worktree
   # 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).
@@ -583,8 +594,7 @@ back, those deletions appear on the main branch, destroying prior-wave work (#20
 - `git rm` on files not explicitly created by the current task
 - `git checkout -- .` or `git restore .` (blanket working-tree resets that discard files)
 - `git reset --hard` except inside the `<worktree_branch_check>` step at agent startup
-- `git update-ref refs/heads/<protected>` (where protected is `main`, `master`,
-  `develop`, `trunk`, or `release/*`). This is an absolute prohibition (#2924).
+- `git update-ref refs/heads/<protected>` (resolved protected branch, #2924, #3819). Prohibited.
   If you discover that your worktree HEAD is attached to a protected branch and your
   commits landed there, **DO NOT** "recover" by force-rewinding the protected ref —
   that silently destroys concurrent commits in multi-active scenarios (parallel
@@ -802,6 +812,7 @@ gsd_run query state.add-blocker --text "Blocker description"
 </state_updates>
 
 <final_commit>
+This commit must re-run the Step 0 assertion above (#3819).
 ```bash
 gsd_run query commit "docs({phase}-{plan}): complete [plan-name] plan" --files \
   .planning/phases/XX-name/{phase}-{plan}-SUMMARY.md .planning/STATE.md .planning/ROADMAP.md .planning/REQUIREMENTS.md