docs(ultraplan-local): remove ultra-cc-architect references (SC-6 doc-rydding)

Step 12 of v3.4.1 plan. Surgical line-by-line generalization of references
to the ultra-cc-architect plugin (no longer publicly distributed):

- CLAUDE.md: 8 hits → "opt-in upstream architect plugin (not bundled)"
- README.md: 9 hits including bare slug at line 646 (removed entirely);
  rephrased to "no longer publicly distributed" with the architecture/
  filesystem slot still supported by /ultraplan-local
- SECURITY.md: 1 hit → generalized "Opt-in upstream architect step"

CHANGELOG.md historical references preserved per brief; appended a
2026-05-04 note at top of [3.0.0] block stating the plugin is no longer
publicly distributed but the architecture/overview.md slot remains
supported for any compatible producer.

The architecture/overview.md filesystem contract (Handover 3, EXTERNAL,
drift-WARN) is unchanged — anyone implementing a compatible producer
can plug in.

361 tests still green (no regressions). doc-consistency pins for
/ultracontinue-local and Handover 7 § Lifecycle still pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/ultraplan-local/SECURITY.md

@@ -53,10 +53,11 @@ phases to detect malformed artifacts before they propagate.
 
 ## Out of scope
 
-- **`ultra-cc-architect` plugin.** Separate plugin with its own
-  `SECURITY.md`. The architecture-discovery validator in this plugin
-  treats `architecture/overview.md` as an external contract (drift-WARN,
-  never drift-FAIL).
+- **Opt-in upstream architect step.** Any external producer of
+  `architecture/overview.md` ships its own security posture. The
+  architecture-discovery validator in this plugin treats
+  `architecture/overview.md` as an external contract (drift-WARN, never
+  drift-FAIL).
 - **LLM output content.** The plugin validates artifact *shape*, not
   artifact *truthfulness*. A plan that passes `plan-validator --strict`
   may still contain hallucinated file paths or unsafe commands; that is