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/shared/pty-rpc-schema.ts

@@ -23,11 +23,16 @@ export type PtyRPCRequests = {
     };
     response: { ok: boolean; error?: string };
   };
-  /** Write raw input bytes (base64-encoded) to a PTY session. */
+  /**
+   * Write input to a PTY session.
+   * `data` is raw UTF-8 text from the user (xterm onData). The pty-client
+   * layer encodes it to base64 before sending to the daemon; this RPC boundary
+   * carries raw text which the Bun handler forwards to PtyClient.writeInput().
+   */
   "pty.write": {
     params: {
       sessionId: string;
-      /** UTF-8 text typed by the user (xterm onData delivers this). */
+      /** Raw UTF-8 text typed by the user (xterm onData delivers this). Encoded to base64 by pty-client before daemon transport. */
       data: string;
     };
     response: { ok: boolean };
@@ -549,11 +554,16 @@ export type PtyRPCRequests = {
     params: { url: string; token: string; label?: string };
     response: { ok: boolean; machineId?: string; error?: string };
   };
-  /** Disconnect from a relay instance. */
+  /** Disconnect from a relay instance (keeps machine in list for reconnect). */
   "remote.disconnect": {
     params: { machineId: string };
     response: { ok: boolean; error?: string };
   };
+  /** Remove a machine entirely — disconnects AND deletes from tracking. */
+  "remote.remove": {
+    params: { machineId: string };
+    response: { ok: boolean; error?: string };
+  };
   /** List all known remote machines with connection status. */
   "remote.list": {
     params: Record<string, never>;