docs(agents): record the riptide-emission and cost invariants

[trick77]

The file was already terse and rule-shaped — nothing to compress. What it lacked were invariants, several of which cost real debugging to establish:

• Emission is best-effort. A slow, rejecting or absent riptide must never fail a webhook or block a review.
• The rollup is claimed before the POST. riptide_emitted_at is stamped by the statement that reads the snapshot, so a rejected emission is never retried — which means a payload riptide's strict schema rejects 422s every rollup, not just the one carrying a new field. Hence: deploy the riptide side first when adding a field. This is exactly what would have bitten us on fix(riptide): report the reviewer account, stop dropping unpriced rollups #186.
• Unknown cost → omit it. Never 0 (understates spend), never drop the rollup (outcome, diff size, tokens and runs need no price). This was the bug fixed in fix(riptide): report the reviewer account, stop dropping unpriced rollups #186: an unpriced model silently withheld whole PR records.
• Declare our own identity. riptide keeps no bot names of its own; undeclared, our comments count as a human's and its review-pickup metric collapses to seconds.
• A None cost fails open — an unpriced model must never block a review; the per-PR cap skips only subsequent auto-runs.
• The disagree mechanic was removed deliberately in migration 009, noted so nobody reinstates it from older docs.

1 331 → 3 151 chars, still well inside budget. Docs only; 649 tests pass.

Candidates I did not add, because they need a proper read of the review path rather than a passing look: the deadline-exceeded summary rules, the "never post an empty summary" rule, and the network-error-does-not-mark-ignored rule. Worth a follow-up pass.