feat(graceful-handoff): 2.0 — migrate to skills/ with disable-model-invocation [skip-docs]

Step 1 of v2.0 plan. Hard cut from commands/ to skills/ per Anthropic
recommendation for new plugins. Frontmatter sets disable-model-invocation:
true and pins model: claude-sonnet-4-6. Docs (README, CLAUDE.md, root
README) deferred to Step 9 per plan.

plugins/ultraplan-local/commands/ultraplan-local.md

@@ -61,11 +61,27 @@ Parse `$ARGUMENTS` for mode flags. Order of precedence:
      Missing: {dir}/brief.md
      ```
    - Set **project_dir = {dir}**, **brief_path = {dir}/brief.md**.
+   - **Validate inputs** (soft mode — warnings do not block, errors do):
+     ```bash
+     # Brief schema sanity check (frontmatter + state machine, soft on body sections)
+     node ${CLAUDE_PLUGIN_ROOT}/lib/validators/brief-validator.mjs --soft --json "{dir}/brief.md"
+
+     # Research briefs (if any) — drift-warn only, none of these block the run
+     [ -d "{dir}/research" ] && \
+       node ${CLAUDE_PLUGIN_ROOT}/lib/validators/research-validator.mjs --soft --dir "{dir}/research" --json
+
+     # Architecture note discovery (EXTERNAL CONTRACT — drift-WARN, never drift-FAIL)
+     node ${CLAUDE_PLUGIN_ROOT}/lib/validators/architecture-discovery.mjs --json "{dir}"
+     ```
+     Each call exits 0 on success or with a structured JSON error report on stderr.
+     Surface any warnings in the user-facing summary at Phase 3, but do not abort.
    - Set **has_research_brief = true** if `{dir}/research/*.md` matches ≥ 1 file.
-   - Set **has_architecture_note = true** if `{dir}/architecture/overview.md` exists.
-     If set, **architecture_note_path = {dir}/architecture/overview.md**. Produced by
-     the optional `/ultra-cc-architect-local` command from the separate `ultra-cc-architect`
-     plugin. Missing file is fine — this is additive discovery, not a requirement.
+   - Read the architecture-discovery JSON output: set **has_architecture_note = true**
+     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.
 
 4. **`--brief <path>`** — extract the brief path. If the file does not exist:
    ```