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/constants/constants.go

@@ -71,6 +71,11 @@ const (
 
 	// FileAccountsJSON is the accounts configuration file in mayor/.
 	FileAccountsJSON = "accounts.json"
+
+	// FileHandoffMarker is the marker file indicating a handoff just occurred.
+	// Written by gt handoff before respawn, cleared by gt prime after detection.
+	// This prevents the handoff loop bug where agents re-run /handoff from context.
+	FileHandoffMarker = "handoff_to_successor"
 )
 
 // Beads configuration constants.