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/cmd/handoff.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/steveyegge/gastown/internal/config"
+	"github.com/steveyegge/gastown/internal/constants"
 	"github.com/steveyegge/gastown/internal/events"
 	"github.com/steveyegge/gastown/internal/session"
 	"github.com/steveyegge/gastown/internal/style"
@@ -192,6 +193,16 @@ func runHandoff(cmd *cobra.Command, args []string) error {
 		style.PrintWarning("could not clear history: %v", err)
 	}
 
+	// Write handoff marker for successor detection (prevents handoff loop bug).
+	// The marker is cleared by gt prime after it outputs the warning.
+	// This tells the new session "you're post-handoff, don't re-run /handoff"
+	if cwd, err := os.Getwd(); err == nil {
+		runtimeDir := filepath.Join(cwd, constants.DirRuntime)
+		_ = os.MkdirAll(runtimeDir, 0755)
+		markerPath := filepath.Join(runtimeDir, constants.FileHandoffMarker)
+		_ = os.WriteFile(markerPath, []byte(currentSession), 0644)
+	}
+
 	// Use exec to respawn the pane - this kills us and restarts
 	return t.RespawnPane(pane, restartCmd)
 }