docs(ultraplan-local): complete external-architect doc rydding [skip-docs]

D2 of post-v3.4.1 stabilisering. Removes 14 plugin-name references from
agents/, commands/, and docs/ tracked files (CLAUDE.md/README.md/SECURITY.md
were ryddet in v3.4.1 commit 6bca3fb).

The external architect plugin was moved out of the public marketplace
2026-05-04 due to ToS concerns around future skill sources. References in
prose are now stale or misleading for public users. The architecture/overview.md
filesystem slot remains available for any compatible producer — discovery
is plugin-agnostic via lib/validators/architecture-discovery.mjs (drift-WARN,
never drift-FAIL).

Files:
- agents/planning-orchestrator.md (1 ref generalized)
- commands/ultraplan-local.md (2 refs generalized; missed by prompt inventory)
- docs/HANDOVER-CONTRACTS.md (4 refs generalized; Handover 3 + stability summary)
- docs/architect-bridge-test.md (deleted; was a public-only bridge checklist)
- docs/subagent-delegation-audit.md (5 refs/rows removed; intervention #5 dropped, recommendation adjusted)

CHANGELOG.md retains historical references (20 occurrences) intentionally.

Verification:
- grep tracked non-CHANGELOG md: 0 references remaining
- npm test: 361/361 pass (baseline preserved)

plugins/ultraplan-local/agents/planning-orchestrator.md

@@ -53,7 +53,8 @@ You will receive a prompt containing:
   auto-discovered `{project_dir}/research/*.md` files and any explicit briefs
   passed via `--research`. Read each brief before launching exploration agents.
 - **Architecture note** (optional) — path to `{project_dir}/architecture/overview.md`
-  produced by the optional `/ultra-cc-architect-local` command. When provided,
+  produced by an external opt-in architect plugin (no longer publicly distributed;
+  the filesystem slot remains available for any compatible producer). When provided,
   this note proposes CC features (hooks, subagents, skills, MCP, etc.) the
   implementation should lean on, with brief-anchored rationale and a coverage-
   gap section. Missing file is fine — this is additive context, not a