fix(run-continuation): dedupe background markers by root session

[Hungdoan565]

Summary

• Aggregate background-task run-continuation markers under a task's root session instead of descendant parent sessions.
• Remove idle-only continuation marker files so completed background-task markers do not leave stale per-session JSON files behind.
• Add regression coverage for descendant root-session marker writes and idle marker cleanup.

Fixes #5742.

Verification

• bun test packages/omo-opencode/src/features/run-continuation-state/storage.test.ts packages/omo-opencode/src/features/background-agent/manager-continuation-marker.test.ts packages/omo-opencode/src/cli/run/continuation-state-marker.test.ts packages/omo-opencode/src/hooks/stop-continuation-guard/index.test.ts packages/omo-opencode/src/hooks/session-notification.test.ts -> 48 pass / 0 fail
• bun test packages/omo-opencode/src/features/background-agent/manager.test.ts packages/omo-opencode/src/features/background-agent/manager-continuation-marker.test.ts -> 198 pass / 0 fail
• bun run typecheck:packages
• git diff --check
• OpenCode QA evidence: .omo/evidence/20260630-run-continuation-session-dedupe-5742/
  • OpenCode 1.17.11 server smoke with local plugin shim
  • OMO plugin entry log and config handler observed
  • /agent returned OMO agents
  • SSE server.connected observed
  • Real OpenCode DB session count unchanged: 3376 -> 3376

---

Summary by cubic

Dedupes run-continuation background markers by writing them only to a task’s root session and removes idle-only markers so files don’t linger. Prevents duplicate per-session JSON and keeps marker storage accurate. Fixes #5742.

• Bug Fixes
  • Aggregate background-task continuation state under the root session instead of descendant parent sessions.
  • Manager computes marker session IDs, active task count, and undelivered wakes per root session.
  • Storage deletes a source when set to "idle"; deletes the file if no sources remain.
  • Tests updated/added for root-session marker dedupe and idle marker cleanup; race tests now expect marker removal.

^{Written for commit 894c580. Summary will update on new commits.}