refactor(daemon): add bounded family drain scheduling

[svarlamov]

Summary

• add a bounded family drain scheduler with four global permits
• preserve single-flight ordering per repository family and schedule a dirty follow-up pass
• retain family errors and completion notifications while safely collecting idle state

Tests

• family serialization, cross-family concurrency, dirty-pass, error, notification, and cleanup unit coverage
• full task test, task lint, task fmt, and task build on the completed stack

Depends on #1979.

[svarlamov]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• fix(daemon): acknowledge checkpoint admission asynchronously #1982
• fix(daemon): decouple trace ingestion from side effects #1981
• refactor(daemon): add bounded family drain scheduling #1980 👈 (View in Graphite)
• fix(daemon): suppress unnecessary agent checkpoint traffic #1979
• main

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