Fix code-modernization plugin: align README with commands, fix pipeline gaps

- modernize-brief: read TOPOLOGY.html (what modernize-map actually
  produces) instead of nonexistent TOPOLOGY.md, and tell the user which
  command produces each missing input.
- README: rewrite the Commands section to match actual command behavior —
  correct output filenames, ordering (brief is the synthesis/approval gate
  after discovery, not the first step), agent attributions, and required
  args. Add a workspace-layout note and an explicit callout that
  modernize-harden edits legacy/, which conflicts with the recommended
  deny rule. Reconcile the Overview and Typical Workflow sequences.
- modernize-assess: generalize the production-runtime overlay step so it
  no longer assumes a specific MCP server/tool; mark it optional. Fix
  app/jcl/ -> legacy/$1/jcl/ for layout consistency.
- modernize-map: make TOPOLOGY.html self-contained (load Mermaid from a
  CDN) so it renders in any browser; drop assumptions about an external
  artifact renderer. Generalize the telemetry annotation note.
- business-rules-extractor agent: fix command cross-reference to the
  actual command name.
- plugin.json: include the brief step in the workflow description.

plugins/code-modernization/.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "code-modernization",
-  "description": "Modernize legacy codebases (COBOL, legacy Java/C++, monolith web apps) with a structured assess → map → extract-rules → reimagine → transform → harden workflow and specialist review agents",
+  "description": "Modernize legacy codebases (COBOL, legacy Java/C++, monolith web apps) with a structured assess → map → extract-rules → brief → reimagine/transform → harden workflow and specialist review agents",
   "author": {
     "name": "Anthropic",
     "email": "support@anthropic.com"