chore(harness): encode post-merge cleanup into ship-pr + session-start

.claude/hooks/session-start.sh

@@ -1,12 +1,33 @@
 #!/bin/bash
-set -euo pipefail
+set -uo pipefail
 
-# Only run in Claude Code on the web; skip on local machines.
+cd "${CLAUDE_PROJECT_DIR:-$(pwd)}"
+
+# --- Stale-branch sensor (always runs, cheap, ~1s with --quiet fetch) ---
+# After `ship-pr` flow / auto-merge with --delete-branch, the remote branch
+# is gone but the local branch lingers. Detect and suggest cleanup so the
+# next session doesn't start on an orphan branch.
+if command -v git >/dev/null 2>&1 && git rev-parse --git-dir >/dev/null 2>&1; then
+  current=$(git symbolic-ref --quiet --short HEAD 2>/dev/null || true)
+  if [ -n "$current" ] && [ "$current" != "main" ]; then
+    # Prune local refs for deleted remote branches. Network call, but cheap
+    # against an already-cloned repo; silenced so we don't spam the session.
+    git fetch --prune --quiet origin 2>/dev/null || true
+    track=$(git rev-parse --symbolic-full-name --abbrev-ref "${current}@{upstream}" 2>/dev/null || true)
+    if [ -n "$track" ] && ! git show-ref --verify --quiet "refs/remotes/${track}"; then
+      echo "[openboot session-start] Branch '$current' tracks '$track' — gone on remote (likely merged)." >&2
+      echo "  Cleanup: git checkout main && git pull && git branch -d $current" >&2
+    fi
+  fi
+fi
+
+# --- Cache warming (Claude Code on the web only) ---
+# Local users don't need this — their caches stay warm across sessions.
 if [ "${CLAUDE_CODE_REMOTE:-}" != "true" ]; then
   exit 0
 fi
 
-cd "${CLAUDE_PROJECT_DIR:-$(pwd)}"
+set -e
 
 echo "[openboot session-start] Go version: $(go version)"
 

.claude/skills/ship-pr/SKILL.md

@@ -108,6 +108,29 @@ Return the PR URL and one sentence on what was queued. Example:
 > PR opened: <url>. Auto-merge enabled — will land once the 6 required
 > checks pass.
 
+### Step 8 — Post-merge cleanup (if already merged)
+
+Immediately re-check whether the PR has already landed — for small PRs
+that hit `MERGEABLE` and pass CI in seconds, `gh pr merge --auto` can
+complete before the user sees the PR URL:
+
+```bash
+gh pr view --json state -q .state
+```
+
+If the output is `MERGED`, the remote branch has been deleted by
+`--delete-branch`. Clean up the local side in the same session so the
+user doesn't have to:
+
+```bash
+git checkout main && git pull --quiet
+git branch -d "$(git symbolic-ref --quiet --short @{-1} 2>/dev/null)"
+```
+
+If the output is `OPEN`, do nothing here. The
+[`session-start.sh`](../../hooks/session-start.sh) stale-branch sensor
+will catch it on the next session and prompt cleanup.
+
 ## What NOT to do
 
 - **Do not skip step 2.** "It's a small change" is exactly when archtest

docs/HARNESS.md

@@ -55,6 +55,8 @@ Three regulation categories:
 | Feedfwd. | Agent conventions | every AI turn | `CLAUDE.md`, `AGENTS.md` |
 | Feedfwd. | Skills | model-loaded | `.claude/skills/*` |
 | Feedfwd. | Session-start hook (warm caches, fetch deps) | every Claude session | `.claude/hooks/session-start.sh` |
+| Feedback (agent) | Stale-branch sensor (suggests cleanup when current branch's upstream is gone) | every Claude session | `.claude/hooks/session-start.sh` |
+| Feedfwd. | `ship-pr` skill — canonical PR flow incl. post-merge cleanup | model-loaded | `.claude/skills/ship-pr/SKILL.md` |
 | Feedback (agent) | `go vet` on edited package | after every Edit/Write/MultiEdit | `.claude/hooks/post-tool-use.sh` |
 | Feedback (agent) | `go vet ./...` + archtest | end of every Claude turn (if .go dirty) | `.claude/hooks/stop.sh` |
 | Maint. | `golangci-lint` on the staged diff | local git pre-commit | `scripts/hooks/pre-commit` |