Announce provider failures to the active Telegram chat

[W1tcherr]

Fixes #77.

A Telegram-initiated run that fails at the provider (quota exhausted, 429, ...) died silently: the extension only forwarded assistant text, and a failed run has none.

This change narrates the failure lifecycle to every chat with a live Telegram turn (same routing as replies, plain text):

• auto_retry_start — first retry announced once per run (error + attempt/max + backoff).
• auto_retry_end — recovery confirmed; exhaustion announces the terminal failure with a /model + /sessions hint.
• retry_fallback_applied — model fallback switch announced.
• agent_end — terminal failure detected via the assistant message with stopReason "error" (covers failures with no retry cycle); skipped when willContinue (a continuation is scheduled, its own events narrate) and deduplicated against the retry-end notice.
• explicit streaming mode opts out, like the idle notify; task subagents never announce.

Plus Outbound.activeTargets()/announce() (gated on live Telegram turns by construction), lastRunError() helper, wiring + unit tests (6 + 3 new), and a Notifications entry in docs/guide.md.

Checks: tsc --noEmit clean, bun test 324 pass / 2 pre-existing env failures (inbox, voice transcription — fail on clean HEAD on Windows too).

[TerrifiedBug]

I pushed two fixes to this branch before merging:

• Keep chat routing and Telegram prompt tools alive across agent_end { willContinue: true }. Only terminal settlement tears them down.
• Stop error lookup at the latest assistant result or user boundary, so a healthy run cannot report an old failure.

The regressions failed on the original revision and pass now. They cover retry exhaustion, successful recovery and its answer in the original topic, prompt-tool preservation, and historical errors. I also integrated #80 and checked that failure notices stay plain text with rich output enabled.

The combined tree passes typechecking and all 380 tests on Bun 1.3.14. The documented minimum is now omp 18.1.16 because this relies on the continuation lifecycle contract.