fix: atomic claim prevents scheduled tasks from executing twice (#657)

* fix: atomic claim prevents scheduled tasks from executing twice (#138)

Replace the two-phase getDueTasks() + deferred updateTaskAfterRun() with
an atomic SQLite transaction (claimDueTasks) that advances next_run
BEFORE dispatching tasks to the queue. This eliminates the race window
where subsequent scheduler polls re-discover in-progress tasks.

Key changes:
- claimDueTasks(): SELECT + UPDATE in a single db.transaction(), so no
  poll can read stale next_run values. Once-tasks get next_run=NULL;
  recurring tasks get next_run advanced to the future.
- computeNextRun(): anchors interval tasks to the scheduled time (not
  Date.now()) to prevent cumulative drift. Includes a while-loop to
  skip missed intervals and a guard against invalid interval values.
- updateTaskAfterRun(): simplified to only record last_run/last_result
  since next_run is already handled by the claim.

Closes #138, #211, #300, #578

Co-authored-by: @taslim (PR #601)
Co-authored-by: @baijunjie (Issue #138)
Co-authored-by: @Michaelliv (Issue #300)
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

* style: apply prettier formatting

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: track running task ID in GroupQueue to prevent duplicate execution (#138)

Previous commits implemented an "atomic claim" approach (claimDueTasks)
that advanced next_run before execution. Per Gavriel's review, this
solved the symptom at the wrong layer and introduced crash-recovery
risks for once-tasks.

This commit reverts claimDueTasks and instead fixes the actual bug:
GroupQueue.enqueueTask() only checked pendingTasks for duplicates, but
running tasks had already been shifted out. Adding runningTaskId to
GroupState closes that gap with a 3-line fix at the correct layer.

The computeNextRun() drift fix is retained, applied post-execution
where it belongs.

Closes #138, #211, #300, #578

Co-authored-by: @taslim (PR #601)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add changelog entry for scheduler duplicate fix

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add contributors for scheduler race condition fix

Co-Authored-By: Taslim <9999802+taslim@users.noreply.github.com>
Co-Authored-By: BaiJunjie <7956480+baijunjie@users.noreply.github.com>
Co-Authored-By: Michael <13676242+Michaelliv@users.noreply.github.com>
Co-Authored-By: Kyle Zhike Chen <3477852+kk17@users.noreply.github.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: gavrielc <gabicohen22@yahoo.com>
Co-authored-by: Taslim <9999802+taslim@users.noreply.github.com>
Co-authored-by: BaiJunjie <7956480+baijunjie@users.noreply.github.com>
Co-authored-by: Michael <13676242+Michaelliv@users.noreply.github.com>
Co-authored-by: Kyle Zhike Chen <3477852+kk17@users.noreply.github.com>

CONTRIBUTORS.md

@@ -9,3 +9,7 @@ Thanks to everyone who has contributed to NanoClaw!
 - [AmaxGuan](https://github.com/AmaxGuan) — Lingfeng Guan
 - [happydog-intj](https://github.com/happydog-intj) — happy dog
 - [bindoon](https://github.com/bindoon) — 潕量
+- [taslim](https://github.com/taslim) — Taslim
+- [baijunjie](https://github.com/baijunjie) — BaiJunjie
+- [Michaelliv](https://github.com/Michaelliv) — Michael
+- [kk17](https://github.com/kk17) — Kyle Zhike Chen