feat(backend): add conversation commit notifier

[sandersonstabo]

Dependency

Immediately above #332 (agent/backend-conversation-delivery-writer-port-19975ca).

Scope

• add an instance-owned, process-wide conversation commit notifier
• retain per-thread wakes with bounded Tokio watch state
• support independent same-thread subscribers, cancellation-safe waits, generation-fenced cleanup, and final-owner closure
• register the focused backend target and Cargo/Bazel test surfaces

Exclusions

No SessionDelivery loop, queue/task, database reads, QUIC/connection ownership, request-handler integration, startup composition, or frontend work.

Verification

Exact-head gate receipt: D:/artisan-editor-gate-logs/backend-conversation-commit-notifier-20260829-gate4/receipt.json

• focused backend + notifier build
• 7/7 focused tests
• fresh production and test Clippy markers
• root format test
• locked/offline Cargo metadata
• exact seven-path hash/status guards
• Bazel shutdown 0

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Summary by cubic

Adds a process-wide conversation commit notifier to the backend so subscribers can await coalesced wake hints without durable state or blocking commit paths.

New Features

• Publishes never block; if the registry lock is busy the hint is dropped and reported as coalesced.
• Per-thread bounded Tokio watch state is shared by same-thread subscribers, each with an independent receiver and cancellation-safe waits.
• Dropping the final notifier owner closes all subscription receivers, and generation-fenced cleanup removes only the last matching entry.
• Not wired into any delivery or request-handler path yet; registers the module, lib export, and Cargo and Bazel test surfaces.

^{Written for commit 17d9694. Summary will update on new commits.}