chore(main): release 0.21.0

[patchmill-release-please-bot]

🤖 I have created a release beep boop

0.21.0 (2026-09-09)

Features

• host: implement Forgejo planning pull request adapter (#215) (6055479)
• host: implement GitHub planning pull request adapter (#213) (2981fd4)
• run-once: publish planning pull request phases (#217) (dc22d38)
• run-once: recover or reset blocked run-once runs (#204) (0b27041)
• workflow: Add durable planning state and pull request workspaces (#216) (7b4856f)
• workflow: define provider-neutral planning pull request foundations (#209) (470891c)

Bug Fixes

• deps: fail fast on stale runtime dependencies (#200) (69bab32)
• deps: reuse pi-subagents upgrade PR (#202) (8a98c2d)
• run-once: preserve ignored state during blocked retry (#212) (cd94670)
• site: remediate dependency advisories (#214) (#218) (da9b143)
• validation: restore baseline quality gates (#208) (7a51191)

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