Bug: excitability decay never auto-runs; hidden side-effect in vault_stats

[raphasouthall]

Summary

Closes the loop on #19. Decay is implemented but not scheduled, and the existing trigger is a surprising read-time side-effect. Result: in a 458-note vault, `dormant=0` and 218 notes (48%) are `never_used`. Additionally, `dormant` is a one-way trap — re-promotion on renewed activity is not implemented.

Root cause

1. No systemd timer. `~/.config/systemd/user/` has timers for harvest, cloud-sync, and vault-health — none for decay. `neurostack decay` is manual-only.
2. Hidden side-effect in `vault_stats`. `src/neurostack/tools/search_tools.py:375-379` calls `run_excitability_demotion` inside `vault_stats`. Stats should be read-only; writes belong in a scheduled path.
3. One-way dormant. `src/neurostack/search.py:472-501` writes `status='dormant'` but never reverses to `active` when score rises again. A renewed note remains "dormant" in `note_metadata.status` despite fresh usage.
4. Two disjoint stores. `note_usage` drives scoring; `note_metadata.status` drives the 1.15× search boost (`search.py:710-740`). They're only reconciled by the decay run — which doesn't run.

Proposed fix

1. Add `neurostack-decay.timer` + `neurostack-decay.service` (daily, e.g., 03:00) in `contrib/systemd/` or whatever path harvest uses.
2. Remove the `run_excitability_demotion` call from `tools/search_tools.py:375-379`. `vault_stats` should be read-only.
3. In `run_excitability_demotion` (`search.py:452-506`), add a re-promotion pass: notes currently `status='dormant'` whose score has risen above threshold get `status='active'`.
4. Optional: surface decay state in `neurostack doctor` — e.g., warn if last decay run is >48h ago.

Expected effect

• `dormant` count becomes meaningful.
• Status column stays in sync with scoring.
• No more hidden writes inside a read endpoint.
• The `×1.15` active boost starts discriminating, which it currently doesn't (every note is active).

Key files

• `src/neurostack/search.py:452-506`
• `src/neurostack/tools/search_tools.py:375-379`
• `src/neurostack/cli/search.py:683-738`
• `~/.config/systemd/user/` (new timer)

[raphasouthall]

Acceptance criteria

• `neurostack-decay.timer` + `.service` installed; `systemctl --user is-active neurostack-decay.timer` returns `active`
• `run_excitability_demotion()` call removed from `tools/search_tools.py:375-379`
• Snapshot test: `vault_stats()` called twice on same DB → no writes between calls (compare WAL and `note_metadata`)
• Re-promotion: note that was dormant and is used again → `status` flips to `active` on next decay run
• With a synthetic usage history stale enough to trigger decay, `dormant > 0` after one run
• `neurostack doctor` warns if last decay run > 48h ago

Dependencies

• Blocks: Design: reconcile neuroscience framing (CREB/LTP/LTD) with actual implementation #35 (the neuroscience reconciliation depends on decay being observable/falsifiable). Generates evidence for tuning Bug: community hierarchy is inverted (fine partition has fewer clusters than coarse) #33.
• Depends on: none. Closes the loop on feat: excitability decay - time-based status demotion #19.

Sequencing

Phase 2 (lifecycle bugs). Prereq for the bigger Phase 4 rework; ship before #35.