Move alert mechanism to the code it constrains; compress alert.md

[nedtwigg]

Stacked on #717. Docs plus code comments; no behavior change.

AGENTS.md → Specs: mechanism that constrains one module lives as a comment at that code, the spec keeps the one-line rule and a Source of truth: pointer. alert.md violated that in several places. This PR moves:

• the OSC 99 key grammar and the OSC 777 field rule → lib/src/lib/terminal-protocol.ts (the OSC 9;4 table stays; it is cross-module behavior);
• the speech-queue bounds (64 pending, 60 s engine timeout, callback-identity revocation, no retry) → lib/src/lib/speech-queue.ts;
• the push-device-list refresh fencing → lib/src/remote/burrow/alert-push.ts (mostly already there);
• two why-clauses → the rationale; the Await section trimmed 911 → 808 words with every bullet kept.

A second reviewer pass restored three conditions the first cut had dropped (the grace window resolves idle unless cancelled; delivery resumes only when adopt_done succeeds; transport.md carries the relay messages, not the alert:await contract). Front matter now names what the spec defers and to whom.

alert.md budget 7050 → 6750. Every Source of truth: symbol verified by spec-lint check 12.

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[dormouse-bot]

Feedback on work in progress, not a merge verdict — mark ready when you want the full review.

The speech-queue bullet keeps the pointer but drops the rule. AGENTS.md → Specs: mechanism constraining a single module lives as a comment at that code, "the spec keeping the one-line rule and a Source of truth: pointer". alert.md now carries only "Queue bounds, the engine timeout, and callback-identity revocation live at SpeechQueue." — no rule survives that sentence, so nothing above the fold says the queue is bounded at all. Compare what this PR did with OSC 777/99, where a bolded one-line rule stayed and only the grammar moved. The numbers (64, 60 s) are the module's mechanism and belong in the TSDoc where you put them; "bound the queue, cancel a stalled attempt, never retry" is the rule.

The ratchet has no room for it. alert.md is at 6,749 words against the new 6,750, so the suggestion below lands at 6,768 and needs node scripts/spec-lint.mjs --ratchet docs/specs/alert.md in the same PR — budget 6,750 → 6,800, still well under the 7,050 it started at.