Commons: add post@1, the broadcast utterance

[cuibonobo]

Summary

Closes #204. Adds org.haverstack/post@1 — the broadcast-speech contract that completes the fourth cell of the artifact/speech × bounded/unbounded 2×2 text-types.md already names, alongside note (kept), message (sent), and article (published).

• docs/commons/post.md (new) — the type file: schema (text required, format, url), the no-date-field rationale (uttering is creating — same reasoning as message@1's missing date field), conventions (parentId threading in-stack, embed/location/syndicated-to cross-type labels, entityId authorship), and the scheduled-posts open question, resolved as "scheduling is the scheduler's state, not the utterance's."
• docs/commons/README.md — post moves out of "deliberately absent from the initial set" into the namespace list and type table as Draft (it now has a concrete intended writer: cuibonobo.com's ephemera stream).
• docs/commons/text-types.md — the two "not yet in the commons" worked-example rows (Foursquare-style check-in, social media post) and the fourth-contract section now point at post.md.
• docs/commons/message.md — cross-references post.md from its "Deliberately excluded" section.
• packages/commons — new POST export, added to defineCommonsTypes's drift/read-compat test coverage.
• .changeset/post-type.md — minor bump for @haverstack/commons.

No @haverstack/core changes needed. The ATProto RFCs (#15, #16) the issue discusses as a would-be blocker are already closed. #203 (settable createdAt for owner-side imports) stays open, but it only blocks migrating an existing dated archive into post@1 — the type itself is fully expressible today, since a post's only meaningful clock is native createdAt.

Spec

No @haverstack/core behavior change — this is a Schema Commons docs + @haverstack/commons package addition, governed by docs/commons/README.md § Governance.

Verification

pnpm run format:check && pnpm run lint && pnpm test && pnpm run build && pnpm run typecheck

All green. One packages/core test (deleteAttachment > ... beyond the first page) failed on the first run with IdGenerationOverflowError — a pre-existing, timing-dependent flake unrelated to this change (confirmed by rerunning it in isolation, and by the fact packages/core isn't touched in this PR); it passed on rerun and the full suite is green.

Notes for reviewers

• Left text-types.md's intro ("Three commons types carry free-form text...") and title unchanged — the doc's own structure treats note/message/article as "the three contracts" and introduces post as an addition via its own "fourth contract" section, so I kept that narrative shape rather than renumbering.
• message.md's changelog got a new "Proposed, amended" entry noting post@1 discharges the broadcast-shape forward-reference it previously carried; message@1 itself stays Proposed (no concrete writer yet) — unrelated to post's status.

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Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

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Generated by Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 024e924

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@haverstack/commons	Minor

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