release: prepare opencode-beads 0.9.3

[snarkipus]

Summary

• describe the plugin as context-preserving atomic delegation with an explicit one-Bead worker boundary
• clarify fork-owned runtime prompting, vendored upstream provenance, and fail-open custom primary-agent lookup
• rewrite README around current installation and ownership boundaries, removing obsolete 0.8.0 migration instructions
• update the documented Beads provenance baseline to v1.2.1
• prepare package identity and changelog for v0.9.3

Validation

• bun run test:docs — 10 passed
• bun test — 74 passed
• bun run typecheck — passed
• bun run test:vendor — 30 prompts validated
• bun run test:provenance — 8 passed
• bun run test:release — 7 passed
• bun run test:package — packed plugin loaded with 29 vendor command files
• ./scripts/test-sync-beads.sh — passed
• actionlint 1.7.12 — passed
• git diff --check — passed
• prepared and verified archive SHA-256: a2f3df5f8bbb1bcdb0c6c03074dbcd61cf62d26df199a9ce356e8ccd22f8289c

Release boundary

This PR does not create a tag, GitHub release, or npm publication. Those remain behind explicit post-merge approval.

Tracks opencode-beads-dvd.