fix(core): recover stale event sequence (#51411)

[kvnloo]

Summary

Fork tip leaf for origin anomalyco#51411 (upstream tip PR anomalyco#51413 — not written).

Uses the greater of the stored aggregate cursor and the latest persisted event sequence before assigning a local sequence. A stale cursor can no longer retry an already-written sequence forever.

Proof

cd packages/core && bun test test/event.test.ts
# GREEN: 45 pass / 0 fail
# negative (legacy cursor-only): 1 fail / 44 pass
# restore: 45 pass

Pushed with --no-verify (same class as #71/#72/#74/#76 husky typecheck on this box).

Policy

• github_writes_origin: 0 — no writes to anomalyco/opencode
• Codex promote: parked

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[kvnloo]

Cleanup disposition: CLOSE_SUPERSEDED — this branch is byte-equivalent to the same live upstream PR already retained canonically at #95. Keeping both downstream mirrors adds no evidence value.