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/commands/modernize-brief.md

@@ -8,8 +8,10 @@ single document a steering committee approves and engineering executes.
 
 Target stack: `$2` (if blank, recommend one based on the assessment findings).
 
-Read `analysis/$1/ASSESSMENT.md`, `TOPOLOGY.md`, and `BUSINESS_RULES.md` first.
-If any are missing, say so and stop.
+Read `analysis/$1/ASSESSMENT.md`, `analysis/$1/TOPOLOGY.html` (and the `.mmd`
+files alongside it), and `analysis/$1/BUSINESS_RULES.md` first. If any are
+missing, say so and stop — they come from `/modernize-assess`, `/modernize-map`,
+and `/modernize-extract-rules` respectively. Run those first.
 
 ## The Brief