fix: routing model mappings and custom endpoint routing

[iBobbyTS]

What changed

• Propagate role model mappings into Claude Code env vars so subagent routing works reliably.
• Stop explicit provider failures from falling back to OpenRouter.
• Fix custom endpoint API key handling, default-provider routing, and model allowlists.
• Update docs to match actual runtime behavior.

Why

• Several routing paths were documented but not actually enforced at runtime.

Validation

• bun test packages/cli/src/claude-runner.test.ts packages/cli/src/proxy-server-routing.test.ts packages/cli/src/providers/custom-endpoints-loader.test.ts packages/cli/src/default-provider.test.ts packages/cli/src/providers/auto-route-default-provider.test.ts
• bun run --cwd packages/cli build
• Additional local tests were run; the only remaining failures were pre-existing model catalog assertions and real-Claude e2e cases that require external state.

Notes

• bun run --cwd packages/cli typecheck hits an existing tsconfig issue referencing packages/core.
• The PR intentionally keeps .skill/ untracked and does not add new documentation structure.