feat(session): Include session ID in PropulsionNudge for /resume picker

PropulsionNudgeForRole now accepts a workDir parameter and reads
session ID from .runtime/session_id to append [session:xxx] to the
nudge message. This enables Claude Code's /resume picker to discover
Gas Town sessions.

(gt-u49zh)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/cmd/mayor.go

@@ -157,7 +157,7 @@ func startMayorSession(t *tmux.Tmux) error {
 	// Send the propulsion nudge to trigger autonomous coordination.
 	// Wait for beacon to be fully processed (needs to be separate prompt)
 	time.Sleep(2 * time.Second)
-	_ = t.NudgeSession(MayorSessionName, session.PropulsionNudgeForRole("mayor")) // Non-fatal
+	_ = t.NudgeSession(MayorSessionName, session.PropulsionNudgeForRole("mayor", townRoot)) // Non-fatal
 
 	return nil
 }