Accept WP-0012-T04: ledger UI treatment

Registration form drops the ledger input entirely -- Trust Service
auto-computes the canonical /phases/{id}/ledger reference at
registration time, no human types it in v0. Schema field itself is
unaffected (still present, still federation-ready). Drill-down shows
only the raw reference + a live-data link when it resolves to this
instance; the existing Ledger entries table on phase_detail.html is
untouched and stays exactly as prominent.

All four T02-T04 decisions now feed T05's spec addendum draft.

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

workplans/TREV-WP-0012-phase-provenance-and-policy-modeling.md

@@ -240,9 +240,11 @@ sequencing (item f, chat 2026-07-30).
 
 ```task
 id: TREV-WP-0012-T04
-status: todo
+status: done
 priority: medium
 human_accept_required: true
+human_accepted_by: Bernd
+human_accepted_at: "2026-08-03"
 state_hub_task_id: "a1cf0180-2dc7-44bb-a34f-99b5e92f5144"
 ```
 
@@ -263,6 +265,28 @@ preference. Decide:
   it automatically now that it is always the ledger's host in Stage 0 —
   distinct from the schema question of whether the field stays.
 
+**Result (accepted 2026-08-03):**
+
+- The Phase registration form (`phase_new.html`) drops the `ledger`
+  input entirely. The Trust Service auto-computes and assigns the
+  canonical `/phases/{id}/ledger` reference at registration time — no
+  human ever types it in v0. The schema field itself is untouched by
+  this decision (still present, still federation-ready for a future
+  Phase whose ledger genuinely lives elsewhere) — this is a Control
+  Plane UI simplification, not a schema change, and needs no
+  addendum-level review of its own.
+- The drill-down view shows only the raw `ledger` reference/URI as
+  stored in the manifest, plus — only when it resolves to this same
+  Trust Service instance — a link to `GET /phases/{id}/ledger`'s live
+  JSON. No new rendering of ledger *data*: `phase_detail.html`'s existing
+  Ledger entries table already covers that and stays exactly as
+  prominent as it is today; this decision only concerns the otherwise-
+  invisible raw reference string, reachable via a small "ledger
+  reference" link/disclosure on `phase_detail.html` rather than a
+  top-level field.
+
+All four T02–T04 decisions now feed T05's spec addendum draft.
+
 ```task
 id: TREV-WP-0012-T05
 status: todo