Add a provider capability matrix (implemented vs listed in UI)

[Railly]

Context

The web UI (apps/web/src/config/providers.ts, ALL_PROVIDERS) lists providers that do not all have implementations in packages/providers/src/providers/. Nothing machine-checkable tracks which formats are implemented vs planned, so UI and code drift silently.

Task

Add a small script (or doc generated by one) that cross-references ALL_PROVIDERS against the actual provider files and outputs a capability table. Commit the table to the README or docs.

Acceptance criteria

• Table lists every provider with status: implemented / planned
• Running the script again after adding a provider updates the table
• Bonus: a CI step that fails when the UI lists a provider with no implementation file

[Railly]

Update after #71: the capability matrix should now live in the README as implemented-vs-planned providers, since the web provider grid moves to /legacy. Cross-referencing ALL_PROVIDERS against packages/providers/src/providers/*.ts still stands as the mechanism.