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CHANGELOG.md retains historical references (20 occurrences) intentionally.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/docs/subagent-delegation-audit.md

@@ -23,7 +23,6 @@ Agent-spawn density per command (nominal):
 |--------------------------|-------------------------------------------------------------------|
 | ultraresearch-local      | ~9–14 (5 local + 4 external + 1 bridge + up to 2 follow-ups)      |
 | ultraplan-local          | ~10 (6 initial + conditional research-scout + up to 3 deep-dives) |
-| ultra-cc-architect-local | 4 (feature-matcher, gap-identifier, critic, scope-guardian)       |
 | ultrabrief-local         | 1–3 (brief-reviewer per iteration, max 3)                         |
 | ultraexecute-local       | 0 (explicit no-agent rule)                                        |
 | ultra-skill-author-local | 3 (concept-extractor → skill-drafter → ip-hygiene-checker)        |
@@ -51,16 +50,11 @@ The main context does the heavy cognitive work after swarm completion:
 - **`commands/ultraresearch-local.md:325–341` (Phase 7 Synthesis):**
   Writes the research brief inline using the template.
 
-- **`commands/ultra-cc-architect-local.md:181+` (Phase 5 Synthesize):**
-  Writes overview.md (6 sections + YAML frontmatter) inline from brief +
-  research + catalog + feature-matcher output.
-
 ### 3. Root cause — v2.4.0 foreground migration
 
 Each command carries a `> **Why foreground?**` block
-(`ultraplan-local.md:330`, `ultraresearch-local.md:192`,
-`ultra-cc-architect-local.md:127`) documenting that the background
-orchestrators were removed because agents spawned from background
+(`ultraplan-local.md:330`, `ultraresearch-local.md:192`) documenting that the
+background orchestrators were removed because agents spawned from background
 orchestrators silently degraded. The swarm-spawn logic was lifted into the
 main context — but so was the synthesis logic the orchestrators used to
 carry. The "summarizer" link is missing.
@@ -76,7 +70,6 @@ are rough estimates based on the size of the phase bodies — not measured.
 | 2 | `plan-writer-agent` — writes plan.md from synthesis + template      | ultraplan Phase 8                   | part of #1   |
 | 3 | `triangulation-synthesizer` — per-dimension local vs external diff + confidence rating | ultraresearch Phase 6               | 25–30%       |
 | 4 | `research-brief-writer` — writes research brief from triangulation output | ultraresearch Phase 7               | part of #3   |
-| 5 | `architecture-writer` — writes overview.md from matcher + gap output | ultra-cc-architect Phase 5          | 15–20%       |
 
 ## Tradeoffs (important)
 
@@ -110,7 +103,7 @@ ultraplan Phase 7)** has the largest ROI. It isolates the heaviest read
 findings document — is small enough to keep resident for Phase 8 planning
 and Phase 9 review.
 
-Interventions #3 and #5 are smaller-scope and lower-risk proofs-of-concept
+Intervention #3 is a smaller-scope and lower-risk proof-of-concept
 that could validate the pattern before touching the main planner.
 
 ## Open questions