chore(activity-def): add legacy-meter-8h-capture for STATE-WP-0073

activity-definitions/legacy-meter-8h-capture.md

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+---
+id: legacy-meter-8h-capture
+name: Legacy-Meter 8h Capture
+type: activity-definition
+version: "1.0"
+enabled: true
+owner: custodian
+governance: custodian
+status: active
+created: "2026-07-09"
+trigger:
+  type: cron
+  cron_expression: "0 */8 * * *"
+  timezone: UTC
+  misfire_policy: skip
+context_sources:
+  - type: state-hub
+    query: legacy_meter_weekly_review
+    required: true
+    params:
+      hours: 8
+      evidence_sinks:
+        - type: state-hub-progress
+          event_type: legacy_meter_8h_capture
+          author: activity-core
+          workplan_id: 44e2e123-9934-4b5e-8b50-1bac548c5b70
+    bind_to: context.legacy_meter_weekly_review
+---
+
+# ActivityDefinition: Legacy-Meter 8h Capture
+
+## Purpose
+
+`STATE-WP-0073` retirement-gate monitoring for `STATE-WP-0070` phase-2 legacy
+retirement. Runs every eight hours (UTC) and posts a compact
+`legacy_meter_8h_capture` progress event with window usage and retirement
+candidates.
+
+Complements the Monday weekly review (`weekly-legacy-meter-review`) which uses a
+7-day window. Seven consecutive zero-usage 8h windows unlock per-key 410
+retirements.
+
+## Trigger
+
+`0 */8 * * *` UTC — 00:00, 08:00, and 16:00 daily. `misfire_policy: skip`.
+
+## State Hub invocation
+
+The `legacy_meter_weekly_review` resolver issues:
+
+- `GET /legacy-meter/weekly-review?hours=8`
+
+On success, activity-core posts progress with `event_type: legacy_meter_8h_capture`
+and probe detail in `detail.probe`.
+
+Manual evidence files (for git-backed audit) remain on the workstation:
+
+```bash
+python ~/state-hub/scripts/capture_legacy_meter_evidence.py
+```
+
+## Output contract
+
+- One State Hub progress event per run (`legacy_meter_8h_capture`)
+- No LLM call; no workplan file edits
+- ActivityRun `context_snapshot` retains review summary for audit