fix: stop IdeaLoom sync from resurrecting a note you deleted in Obsidian

[atomantic]

Summary

• Deletion is a decision, not drift. A previously-exchanged list whose vault note is gone now reports a distinct missing outcome and writes nothing. Before, exportOne re-created the note on the next write, resurrecting data the user deleted on purpose. Recreating it takes an explicit request (recreateMissing), surfaced as a "Recreate N deleted notes" action that only appears after an exchange reported one.
• unavailable ≠ missing. The import-side probe uses existsSync rather than the scan result, so an iCloud note that has not been downloaded stays unavailable and is never offered as a recovery candidate.
• Auto-sync is now real, and is the least powerful path in the feature. autoSync was a stored setting nothing read. It now debounces a burst of edits into one vault write, re-reads both toggles when the write fires (so turning either off cancels queued work), and calls the exchange with the fail-closed defaults — it cannot delete a note, recreate one, or resolve a conflict.
• No import/export feedback loop. Only the two local-write routes schedule an export; import and sync deliberately do not. Additionally, an export whose rendered Markdown already hashes equal to the note on disk is skipped rather than rewritten, so a freshly imported list cannot export itself back.
• UI: the integration settings (enable → pick vault → enable auto-sync) are reachable for the first time; updateIdeaLoomSettings previously had no caller. The exchange summary now names every outcome including missing.
• Docs: docs/features/brain-system.md gains the base-hash conflict table, the deletion contract, and the auto-sync limits — replacing the paragraph that deferred all three to this slice. docs/API.md names the sync body flags; docs/STORAGE.md records the conflict/deletion behavior and backup coverage.

Closes #5339.

Test plan

• server/services/idealoomAutoSync.test.js (new): debounce coalescing, fail-closed on each of the three toggles, toggles re-read at fire time, recreateMissing never passed, a failed export not crashing the process, unresolved outcomes warned. Fake timers throughout — no production sleeps.
• server/services/idealoomObsidian.test.js: externally deleted note → missing with nothing written, then recreated under recreateMissing; a deleted note surfaced on import without dropping the local list; a note already matching the list is skipped, not rewritten.
• server/routes/brain.test.js: recreateMissing defaults to false and threads through when set; auto-sync is scheduled by create/update only — never by delete, import, or sync.
• client/src/components/brain/IdeaLoomLists.test.jsx: the auto-sync toggle is unreachable until sync is enabled and a vault is chosen; recovery is offered only after an exchange reports a deleted note, and the plain export does not ask for a recreate.
• Full server (35,453) and client (10,223) suites pass; client biome lint clean; API route catalog regenerated.