docs+ci: prep beta docs and prerelease publish

[leoafarias]

Summary

• update llms.txt with the latest @AckType discriminated union feature and constraints from Add @AckType discriminated union extension generation #82
• refresh stale install snippets from 1.0.0-beta.6 to 1.0.0-beta.7 in package READMEs
• tighten ack_generator unreleased changelog notes for next beta release
• allow prerelease tags (for example v1.0.0-beta.8) in publish workflow

Notes

• changelog link-only entries are already automated by the existing melos version pre-commit hook (No changes required for packages/ack/CHANGELOG.md
  No changes required for packages/ack_annotations/CHANGELOG.md
  No changes required for packages/ack_generator/CHANGELOG.md
  No changes required for packages/ack_firebase_ai/CHANGELOG.md
  No changes required for packages/ack_json_schema_builder/CHANGELOG.md) for all publishable packages, including ack_generator

Validation

• verified no active README/llms install snippets still reference 1.0.0-beta.6
• verified workflow trigger patterns now include prerelease tags

[docs-page]

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docs.page/btwld/ack~83

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