Push Git notes asynchronously per repository family

[svarlamov]

Summary

• Capture immutable push destinations from trace2 transport events without adding Git work to ingestion.
• Queue Git-notes delivery only after the user push completes.
• Serialize pushes per repository family, coalesce repeated destinations, and bound ingress, per-command, and per-family queues.
• Deliver notes to every distinct traced push URL, including mirrored remotes.
• Add structured lifecycle, overflow, failure, and timeout logging.
• Cover prompt return, retries, explicit, relative, and mirrored destinations, HTTP isolation, and worker concurrency with TestRepo-based and focused trace regressions.

Verification

• task fmt
• task lint
• task build
• task test CARGO_TEST_ARGS="--lib" — 2,197 passed
• task test CARGO_TEST_ARGS="--test notes_sync_regression" — 26 passed

Stack 2/2; depends on #2116.

Fixes #2099.