fix: remember_event atomicity + dead code cleanup (post-porting review)

[marlian]

Summary

Addresses Diana post-porting review on 8dddddf (the Node→Go port that landed via PR #2). Findings:

• R4 — atomic remember_event (state-concurrency). CreateEvent wrote the event row on *sql.DB directly, before the observation-loop transaction opened. Any mid-loop DB error (SQLITE_BUSY, FK violation, disk full) left an orphan event row with zero attached observations. Fix: CreateEvent now accepts dbtx; the handler opens a single transaction covering event insert + observation loop. Rollback discards the whole unit.
• R1 — scanEntityRecord dead code removed from events.go. Defined but never called.
• R2 — EventTypeFilter field removed from ToolArgs. Declared but never read — silent no-op contract drift for any JSON caller passing event_type_filter. The existing EventType field already serves recall_events.
• R3 — real-client proof was archaeology. The binary is already wired as the MCP server backing Claude Code (via ~/.claude.json + governor env files), Kilo, and Codex. Live tool calls (remember_batch, recall_entity) succeeded during this very review session. Removing the stale P1 entry from OPERATIONS.md.

Invariant added to OPERATIONS.md: remember_event must be atomic — event row and all attached observations commit together or not at all.

Test plan

• TestRememberEventAtomicityOnMidLoopFailure installs a BEFORE INSERT trigger on entities that RAISE(ABORT) on a sentinel value. Pre-fix: before=0 after=1 (orphan event row left behind). Post-fix: passes.
• Full go test ./... suite green on darwin/arm64.
• CI matrix (ubuntu / macos / windows) on this branch.

Commits

1. fix: make remember_event atomic against mid-loop DB errors
2. chore: remove dead code flagged by Diana review
3. docs: close R3 real-client debt — binary is already in production

[Copilot]

Pull request overview

Improves correctness of remember_event by ensuring event creation and observation attachments commit atomically, while also removing unused/dead code surfaced during post-port review.

Changes:

• Wrap remember_event event insert + observation attachment loop in a single transaction to prevent orphan event rows on mid-loop failure.
• Refactor CreateEvent to accept a transaction-capable interface (dbtx) and remove unused scanEntityRecord.
• Add a regression test proving rollback behavior on injected mid-loop failure; document the invariant in OPERATIONS.md.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
internal/store/tools.go	Makes remember_event transactional end-to-end; removes unused EventTypeFilter arg field.
internal/store/parity_test.go	Adds TestRememberEventAtomicityOnMidLoopFailure to prevent orphan events/entities on failures.
internal/store/events.go	Updates CreateEvent to accept dbtx; removes dead scanEntityRecord.
OPERATIONS.md	Documents the remember_event atomicity invariant with a concrete test reference.