chore(release): 0.9.0

[benvinegar]

Release prep for 0.9.0 — consumes all pending changesets and bumps the version.

Generated via npm run release:version. No code changes; only CHANGELOG.md, package.json, and consumed .changeset/ fragments.

Validation (all green):

• npm run format:check ✓
• npm run lint ✓
• npm run typecheck ✓
• npm test — 333 pass ✓
• npm run test:e2e — 68 pass (chromium + webkit) ✓

Highlights in this release:

• post / surface vocabulary across the wire, MCP, viewer, and prose (additive — legacy aliases kept)
• ~/.sideshow/ default data dir with one-time migration (fixes sudo-install EACCES + upgrade data loss)
• open-as-image surface action (Workers deploys)
• sideshow --version with update check
• SSE/long-poll connection cap, SQLite mkdir guard, submit-time diff/mermaid validation, mobile viewer improvements
• Host-contract change (embedders): identity.accountSlug → identity.workspaceSlug

After merge, cut the tag on main to trigger publish:

git checkout main && git pull
git tag v0.9.0 && git push origin v0.9.0