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CHANGELOG.md retains historical references (20 occurrences) intentionally.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/commands/ultraplan-local.md

@@ -80,8 +80,9 @@ Parse `$ARGUMENTS` for mode flags. Order of precedence:
      if `found == true`. The discovery module emits warnings if the file lives at a
      non-canonical path (e.g. `architecture-overview.md`); preserve them for the
      user-facing summary. If set, **architecture_note_path = {result.overview}**.
-     Produced by the optional `/ultra-cc-architect-local` command from the separate
-     `ultra-cc-architect` plugin. Missing file is fine — additive discovery, not required.
+     Produced by an external opt-in architect plugin (no longer publicly distributed;
+     the filesystem slot remains available for any compatible producer). Missing file
+     is fine — additive discovery, not required.
 
 4. **`--brief <path>`** — extract the brief path. If the file does not exist:
    ```