fix: agent reliability — no restart on channel-add, visible dead-letter notice

[wesbillman]

Two focused fixes for the dominant agent-reliability failure modes discussed in #agent-reliability: turns silently dying when a running agent is added to a channel, and dead turns leaving channels waiting forever with no visible signal.

1. Don't restart running local agents on channel-add (desktop)

Adding a running local agent to a channel did stopManagedAgent → startManagedAgent to pick up the new subscription — killing any in-flight turn. That branch dates to #86, before buzz-acp learned dynamic subscriptions; the harness now subscribes to new channels live via member-added notifications, and the desktop doesn't set BUZZ_ACP_CHANNELS, so nothing blocks the dynamic path.

The fix is mostly deletion: remove the restart branch in attachManagedAgentToChannel plus the restarted result plumbing and stale UI copy.

2. Visible notice when a turn is dead-lettered (buzz-acp, buzz-agent)

When retries were exhausted, the queue dead-lettered the batch with only an ERROR log — the channel never learned the turn died. Now:

• EventQueue::requeue returns the exhausted batch instead of discarding it.
• handle_prompt_result posts a short kind:9 into the triggering thread: "⚠️ I couldn't process the last request after multiple retries (<reason>). Please re-send if it's still needed."
• Transient failures still retry quietly with exponential backoff — only final dead-letter is surfaced, so no noise.
• The notice is best-effort (5s timeout, errors logged and swallowed) and posted from a spawned task so it can never stall the main loop.

Also raises the buzz-agent LLM call timeout default 120s → 240s so slow-but-successful LLM calls stop being classified as failures and burning retries.

Testing

• cargo test: buzz-acp 405 passed, buzz-agent 122 passed (includes a new dead-letter queue test)
• cargo clippy clean; root + desktop-tauri rustfmt clean
• desktop: pnpm test 1494 passed; typecheck clean
• Playwright smoke: agents / profile / active-turn-resilience / profile-active-turn / smoke specs — 22 passed

Co-authored-by: Brain 21994759fc7a6fa6b965551d35cfd7897d262f2495467f2d78694ddcfa6a5c7e@sprout-oss.stage.blox.sqprod.co