docs: specify forward poll cursors

[shivros]

Summary

Adds the frozen OpenSpec contract for COD-456's agent-discoverable forward polling.

• Preserves existing list_messages / list_threads array responses unchanged.
• Adds additive generated poll_messages / poll_threads operations with opaque forward cursors.
• Defines safe bootstrap, total source ordering, bounded continuation, AEAD cursor binding/rotation, email UIDVALIDITY behavior, and explicit thread-change semantics.

Verification

• git diff --check — pass
• OpenSpec artifact structural validation — pass
• openspec validate add-forward-poll-cursors --strict — unavailable: openspec is not installed on this host

Implementation gate

This is a spec-only PR. T1 requires review/approval and merge before implementation tickets/PRs begin.

Closes COD-456

[shivros]

🤖 Automated Review Panel

Reviewer A — BLOCK → resolved

Initial review found four contract gaps: incompatible array-to-envelope replacement, undefined empty bootstrap, unsafe bounded cursor advancement, and unspecified query binding. The spec now keeps existing list arrays unchanged; introduces additive poll_* pages; requires a non-null bootstrap checkpoint even for empty sources; advances only through the final emitted item; and enumerates operation/principal/instance/thread bindings.

Follow-up review found one remaining ordering gap. Resolved: the contract now requires an immutable, totally ordered source position with a deterministic tie-breaker and a consistent source snapshot. Email uses UID within UIDVALIDITY.

Reviewer B — BLOCK → resolved

Initial review additionally found bootstrap ambiguity, thread-update ambiguity, cursor confidentiality risk, misleading no-I/O epoch validation, missing generic capability/error semantics, and missing generated-surface compatibility policy. The final spec defines zero-item high-water bootstrap; ordered thread-change observations (duplicates allowed); AEAD encrypted, size-limited, key-rotated tokens; pre-I/O outer validation plus minimal metadata read for UIDVALIDITY; generic ForwardPolling capability/stable errors; and additive HTTP/MCP/CLI operations preserving existing array contracts.

Follow-up review: APPROVE.

Panel result: APPROVE after fixes. This PR deliberately contains the frozen contract only; implementation is gated on its merge.