refactor(api): establish provider extension groundwork

[hanityx]

Summary

• Establish the api-ts provider domain boundary and registry/capability split.
• Move provider-specific facts toward adapters while keeping cross-provider behavior in services.
• Add dependency boundary rules for app/domain/lib/platform layering.

Stack

1. This PR: provider extension groundwork
2. Provider policy/test split
3. Route and system boundary split
4. Runtime regressions and QA gates
5. Unsupported provider cleanup and contributor docs

Validation

• Full stack validation previously run: corepack pnpm qa:provider PASS, corepack pnpm qa:pr PASS.
• Final hygiene pass after docs/test fixture cleanup: api-ts tests 43 files / 273 tests PASS, TUI tests 69 PASS, public markdown hygiene PASS.