fix(electrobun): complete all 16 Codex #3 findings

CRITICAL:
- Message persistence race: snapshot batchEnd before async save
- Double-start guard: startingProjects Set prevents concurrent launches
- Symlink path traversal: fs.realpathSync() in path-guard.ts
- Relay false success: connect() returns { ok, machineId, error }

HIGH:
- Session restore skips if active session exists
- Remote remove: new RPC, cleans backend map
- Task board poll token: stale responses discarded after drag-drop
- Health concurrent tools: toolsInFlight counter (was boolean)
- bttask transactions: delete wraps comments+task, addComment validates
- PTY buffer cleared on reconnect
- PTY large paste: chunked String.fromCharCode (8KB chunks)
- Sidecar max line: 10MB limit prevents unbounded memory
- btmsg authorization: group validation, channel membership checks

MEDIUM:
- Session retention: max 5 per project, purgeSession/untrackProject
- Relay IPv6: URL parser replaces string split
- PTY schema: fixed misleading base64 comment

ui-electrobun/src/mainview/agent-store.svelte.ts

@@ -259,6 +259,8 @@ function ensureListeners() {
       }
       persistMessages(session);
       scheduleCleanup(session.sessionId, session.projectId);
+      // Fix #14 (Codex audit): Enforce max sessions per project on completion
+      enforceMaxSessions(session.projectId);
     }
   });