feat(adf): migrate should classify rules into on-demand modules, not just core.adf

[SmartBrandStrategies]

Summary

charter adf migrate extracts rules from agent config files (agents.md, .cursorrules) and merges everything into core.adf, even when content clearly belongs in on-demand modules like frontend.adf or backend.adf.

Example

Given an agents.md with mixed content:

React components: PascalCase.tsx
Hooks/utilities: camelCase.ts
Hono route composition for worker/backend APIs
npm run db:local creates the local D1 schema

The migration classifies all of these as CONSTRAINTS → core.adf:

"React components: PascalCase.tsx"        → CONSTRAINTS [advisory]  core.adf
"Hono route composition for worker/..."   → CONSTRAINTS [advisory]  core.adf
"npm run db:local creates the local..."   → CONSTRAINTS [advisory]  core.adf

Expected Behavior

The migration should route rules to the appropriate on-demand module when the manifest declares them:

Rule	Target
React component naming conventions	frontend.adf
Vite dev server commands	frontend.adf
Hono route patterns	backend.adf
D1 schema/migration commands	backend.adf
TypeScript strictness, commit conventions	core.adf

Suggested Approach

Use the ON_DEMAND module metadata from manifest.adf (which already includes keyword hints like react, css, ui for frontend and api, node, db for backend) to classify migrated rules into the best-fit module rather than defaulting everything to core.

This would make the on-demand modules useful out of the box instead of leaving them as empty scaffolds after migration.

[SmartBrandStrategies]

Update: The routing logic already exists in `content-classifier.ts` on both main and v0.5.0 — keyword-based matching routes to `frontend.adf` / `backend.adf` / `core.adf`:

```ts
if (/\b(design.system|ui|frontend|css|component|react|vue|svelte)\b/.test(lower)) {
return 'frontend.adf';
}
if (/\b(api|backend|deploy|server|database|db|endpoint)\b/.test(lower)) {
return 'backend.adf';
}
return 'core.adf';
```

However, when tested with v0.4.2, all 23 rules routed to `core.adf` despite rules containing keywords like "React", "Hono", "D1", "db:local". The `applyMigrationToModule()` function does handle missing files (creates an empty ADF doc), so that's not the blocker.

Possible causes:

1. The classifier regex isn't matching because the rule text is e.g. `"React components: PascalCase.tsx"` where "React" is at the start — the `\b` word boundary should match this. Need to verify the `lower` variable is actually the item text and not a section heading.
2. The published 0.4.2 npm package may not include the latest `content-classifier.ts` from main (build artifact drift).

This needs investigation — the code looks correct but the runtime behavior doesn't match.