fix(assistant): eliminate update-bulletin wake-resolver race + clean up orphans

[siddseethepalli]

Summary

• Primary race fix: runUpdateBulletinJobIfNeeded() is now dispatched AFTER DaemonServer.start() resolves, which is where registerDefaultWakeResolver() runs. The previous dispatch site ran concurrently with server startup, so the wake fired before the resolver was in place and silently returned {invoked: false}. The dispatch remains fire-and-forget (void import(...).then(...).catch(...)), preserving the 'daemon must never block startup' invariant.
• Belt-and-suspenders cleanup: even with the race fix, wakeAgentForOpportunity() can return {invoked: false} for other reasons (conversation-not-found, future regressions). The job now detects this and deletes the orphan background conversation it just bootstrapped, so we don't leak DB rows and LLM title-generation calls.
• Added a regression test that asserts both branches (race-case cleanup + normal-case no cleanup).

Part of plan: updates-md-background-job.md (round 2 post-review fix)

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[devin-ai-integration]

Devin Review found 3 potential issues.

View 2 additional findings in Devin Review.

[siddseethepalli]

Addressed in #26473