Migrate dashboard LogQueryWidget widgets to EMF metrics

> **Follow-up from [PR #88](https://github.com/aws-samples/sample-autonomous-cloud-coding-agents/pull/88)** — long-term observability cleanup; not blocking but worth tracking.

## Functional description

ABCA's CloudWatch dashboard has two distinct kinds of widgets:

- **EMF metric widgets** (added in PR #88, Chunk 8b): driven by `ApprovalMetricsPublisher` extracting structured metrics from `TaskEventsTable` DDB streams. Refresh every ~1 minute. Cheap. Show approval latency by outcome, etc.
- **`LogQueryWidget` widgets** (older): run CloudWatch Logs Insights queries against the `TaskEventsTable` stream → CloudWatch log group on every dashboard load. Slower (each query takes seconds). More expensive (CWLI charges per GB scanned). Limited by CWLI sample-window semantics.

The PR #88 widgets prove the EMF pattern works for ABCA's use case. **The remaining `LogQueryWidget` widgets are now technical debt** — they're the older pattern, and once a third widget needs a metric the old query produces, the migration becomes worth doing.

**User-visible impact (today):**

- Dashboard takes longer to load than necessary.
- Some metrics are stale by the CWLI sample window (often 5+ minutes).
- AWS bill has avoidable CWLI scan costs.

**User-visible impact (in 6 months if not done):**

- Adding new dashboard widgets gets harder because every new metric needs its own producer (publisher → EMF) AND a parallel `LogQueryWidget` for backfill / historical comparison.
- Two patterns to maintain instead of one.

## Technical context

**Which widgets use which pattern:** needs an audit. Likely:

- `cdk/src/constructs/task-dashboard.ts` (or similar) — defines the dashboard. Search for `LogQueryWidget` to find the legacy ones.
- The EMF-side definitions are likely in the same file or an adjacent `cedar-hitl-dashboard.ts`.

**Migration shape per widget:**

1. Identify what the legacy `LogQueryWidget` measures (e.g. "task throughput by outcome over time").
2. Add the matching EMF metric to `ApprovalMetricsPublisher` (or extend if conceptually similar).
3. Add the EMF widget to the dashboard.
4. Remove the legacy `LogQueryWidget`.

**Why this is P3:**

- Today's `LogQueryWidget`s **work**, just slower.
- The migration touches the publisher Lambda — a load-bearing component. Risk of regression.
- No customer is asking for this; it's hygiene.

**When to escalate to P2:**

- If the dashboard sees a third active maintainer (the cost of two patterns shows up in PR review friction).
- If the AWS bill grows enough that the CWLI scan costs matter.
- If a new widget needs both EMF (live) and historical backfill — that's the natural moment to migrate.

## Proposed approach

**Phase 1 — audit + plan (this issue):**
- List every `LogQueryWidget` in the dashboard config.
- For each, note what metric it measures and whether the publisher already emits an equivalent EMF metric.
- Estimate effort per widget (most should be < 1 hour).

**Phase 2 — migrate one widget as proof:**
- Pick the lowest-blast-radius widget.
- Land the migration as a small PR.
- Validate: the new widget shows the same data as the old one (run both side by side for a week before deleting the old).

**Phase 3 — finish:**
- Migrate remaining widgets in batches.

Recommend doing **phase 1 only as part of this issue**; phases 2 + 3 become their own issues once the audit reveals scope.

## Acceptance criteria

- [ ] An audit document (or comment on this issue) lists every `LogQueryWidget` in the dashboard, what it measures, and whether an EMF equivalent exists
- [ ] For each LegacyWidget without an EMF equivalent, the audit notes whether one is feasible (most should be)
- [ ] Estimated effort per widget is captured so the next person can prioritize

## Out of scope

- Actually migrating widgets (those become per-widget follow-up issues / PRs).
- Adding new metrics that don't have a legacy `LogQueryWidget` counterpart.
- Cross-account dashboard syndication.

## References

- `cdk/src/constructs/task-dashboard.ts` (or wherever the dashboard lives — needs locating)
- `cdk/src/constructs/cedar-hitl-dashboard.ts` (PR #88 EMF dashboard, the migration target shape)
- `docs/design/CEDAR_HITL_GATES.md` §10 (dashboard design)
- Companion: #4, #5 (DLQ + EMF throttle alarms — same observability area)