feat: implement priming subsystem improvements

Phase 1 of dynamic priming subsystem:

1. PRIME.md provisioning for all workers (hq-5z76w, hq-ukjrr Part A)
   - Added ProvisionPrimeMD to beads package with Gas Town context template
   - Provision at rig level in AddRig() so all workers inherit it
   - Added fallback provisioning in crew and polecat managers
   - Created PRIME.md for existing rigs

2. Post-handoff detection to prevent handoff loop bug (hq-ukjrr Part B)
   - Added FileHandoffMarker constant (.runtime/handoff_to_successor)
   - gt handoff writes marker before respawn
   - gt prime detects marker and outputs "HANDOFF COMPLETE" warning
   - Marker cleared after detection to prevent duplicate warnings

3. Priming health checks for gt doctor (hq-5scnt)
   - New priming_check.go validates priming subsystem configuration
   - Checks: SessionStart hook, gt prime command, PRIME.md presence
   - Warns if CLAUDE.md is too large (should be bootstrap pointer)
   - Fixable: provisions missing PRIME.md files

This ensures crew workers get Gas Town context (GUPP, hooks, propulsion)
even if the gt prime hook fails, via bd prime fallback.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/beads/beads.go

@@ -599,3 +599,87 @@ func (b *Beads) IsBeadsRepo() bool {
 	info, err := os.Stat(beadsDir)
 	return err == nil && info.IsDir()
 }
+
+// primeContent is the Gas Town PRIME.md content that provides essential context
+// for crew workers. This is the fallback if the SessionStart hook fails.
+const primeContent = `# Gas Town Worker Context
+
+> **Context Recovery**: Run ` + "`gt prime`" + ` for full context after compaction or new session.
+
+## The Propulsion Principle (GUPP)
+
+**If you find work on your hook, YOU RUN IT.**
+
+No confirmation. No waiting. No announcements. The hook having work IS the assignment.
+This is physics, not politeness. Gas Town is a steam engine - you are a piston.
+
+**Failure mode we're preventing:**
+- Agent starts with work on hook
+- Agent announces itself and waits for human to say "ok go"
+- Human is AFK / trusting the engine to run
+- Work sits idle. The whole system stalls.
+
+## Startup Protocol
+
+1. Check your hook: ` + "`gt mol status`" + `
+2. If work is hooked → EXECUTE (no announcement, no waiting)
+3. If hook empty → Check mail: ` + "`gt mail inbox`" + `
+4. Still nothing? Wait for user instructions
+
+## Key Commands
+
+- ` + "`gt prime`" + ` - Get full role context (run after compaction)
+- ` + "`gt mol status`" + ` - Check your hooked work
+- ` + "`gt mail inbox`" + ` - Check for messages
+- ` + "`bd ready`" + ` - Find available work (no blockers)
+- ` + "`bd sync`" + ` - Sync beads changes
+
+## Session Close Protocol
+
+Before saying "done":
+1. git status (check what changed)
+2. git add <files> (stage code changes)
+3. bd sync (commit beads changes)
+4. git commit -m "..." (commit code)
+5. bd sync (commit any new beads changes)
+6. git push (push to remote)
+
+**Work is not done until pushed.**
+`
+
+// ProvisionPrimeMD writes the Gas Town PRIME.md file to the specified beads directory.
+// This provides essential Gas Town context (GUPP, startup protocol) as a fallback
+// if the SessionStart hook fails. The PRIME.md is read by bd prime.
+//
+// The beadsDir should be the actual beads directory (after following any redirect).
+// Returns nil if PRIME.md already exists (idempotent).
+func ProvisionPrimeMD(beadsDir string) error {
+	primePath := filepath.Join(beadsDir, "PRIME.md")
+
+	// Check if already exists - don't overwrite customizations
+	if _, err := os.Stat(primePath); err == nil {
+		return nil // Already exists, don't overwrite
+	}
+
+	// Create .beads directory if it doesn't exist
+	if err := os.MkdirAll(beadsDir, 0755); err != nil {
+		return fmt.Errorf("creating beads dir: %w", err)
+	}
+
+	// Write PRIME.md
+	if err := os.WriteFile(primePath, []byte(primeContent), 0644); err != nil {
+		return fmt.Errorf("writing PRIME.md: %w", err)
+	}
+
+	return nil
+}
+
+// ProvisionPrimeMDForWorktree provisions PRIME.md for a worktree by following its redirect.
+// This is the main entry point for crew/polecat provisioning.
+func ProvisionPrimeMDForWorktree(worktreePath string) error {
+	// Resolve the beads directory (follows redirect chain)
+	beadsDir := ResolveBeadsDir(worktreePath)
+
+	// Provision PRIME.md in the target directory
+	return ProvisionPrimeMD(beadsDir)
+}