docs: rebuild code-derived documentation

[pourdaavar]

Summary

• replace legacy READMEs with code-derived package guides
• add focused architecture, development, package, and integration documentation
• remove obsolete planning and duplicated documentation

Verification

• pnpm test
• pnpm exec prettier --check
• pnpm exec cspell --no-must-find-files
• git diff --check

[changeset-bot]

⚠️ No Changeset found

Latest commit: 4d13022

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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