feat: add pty_wait tool

[FAZuH]

Summary

• Add a pty_wait tool that blocks until a PTY session finishes running, with an optional timeoutSeconds.
• Returns a <pty_waited> block with final status, exit code/signal, output line count, and the last 20 lines of output; returns a <pty_wait_timeout> block instead of blocking forever when the timeout elapses.

Motivation

Exit notifications (notifyOnExit → <pty_exited>) require the agent to go idle and wait for the injected message. This is unreliable when:

• A subagent goes idle waiting for <pty_exited> and the parent interprets idleness as task completion.
• A /goal plugin auto-resumes idle agents, so an agent idling to wait for <pty_exited> gets repeatedly resumed.

pty_wait gives the agent a synchronous blocking alternative in both scenarios.

Changes

• src/plugin/pty/tools/wait.ts: new tool: one-shot session-update callback raced against an optional timeout (Promise.race), closing the register/exit race window
• src/plugin/pty/tools/wait.txt: tool description
• src/plugin.ts: registers pty_wait alongside the other pty_* tools
• src/plugin/pty/manager.ts:: iterate a snapshot of sessionUpdateCallbacks so a callback removing itself mid-iteration can't skip other subscribers (e.g. the web UI's WS broadcast)
• README.md: tools table row + a "Wait for a session to finish" section
• test/pty-tools.test.ts: ptyWait suite: already-exited, tail window, callback-driven exit, timeout, exit-signal, not-found, plus callback-leak hygiene

Verification

• bun lint
• bun format
• bun run typecheck
• bun unittest (73 passed, 1 skipped)