feat(legacy-meter): meter MCP aliases and fix brief workstream_id label (STATE-WP-0070 T03/T04)

T03: MCP alias retirement gate was unstartable — deprecation notices existed
but no mcp:* interfaces were ever registered or metered. Add
_meter_legacy_mcp() posting /legacy-meter/usage (kind mcp_tool, component
state-hub.mcp) from create_workstream, list_workstreams, update_workstream,
update_workstream_status, and state://workstreams/{topic_slug}; register all
five phase-3 backlog keys so zero-window streaks accrue from today.

T04: trace residual POST /progress/ workstream_id (3 calls/8h, unknown) to
.custodian-brief.md labelling workplans "workstream_id:"; brief generator now
prints "workplan_id:". Briefs refresh on each repo's next fix-consistency run.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/workplan-terminology-legacy-retirement-backlog.md

@@ -57,6 +57,10 @@ Risk order: REST > MCP > events > dashboard prose > internal identifiers.
 | 3 | `mcp:update_workstream_status` | `update_workplan_status` | MCP | T03 |
 | 3 | `mcp:list_workstreams` | `list_workplans` | MCP | T03 |
 | 3 | `state://workstreams/{topic_slug}` | `state://workplans/{topic_slug}` (proposed) | MCP resource | T03 |
+
+Phase-3 MCP keys metered + registered 2026-07-10 (STATE-WP-0070 T03):
+`_meter_legacy_mcp()` records usage from the alias tools/resource; zero-window
+streaks accrue from registration.
 | 4 | ~~`event_subject:org.statehub.workstream.completed`~~ **retired** | `org.statehub.workplan.completed` | Event | T05 ✓ |
 | 5 | Dashboard nav label `Workstreams` | `Workplans` (URL compat retained) | Prose | T02 |
 | 5 | `dashboard/src/index.md` user-facing copy | workplan-first strings | Prose | T02 |