feat(pi-openrouter): replace runtime provider override with models.json sync CLI

The extension overrode pi's built-in openrouter provider at runtime, which
replaced the full catalog with a curated list, dropped openRouterRouting
compat, and only existed in processes that load extensions — headless
children and subagents resolved models differently and broke.

pi core supports per-model api/baseUrl in models.json (verified against
completions, responses, and messages surfaces), so the entire routed
provider is expressible as config. pi-openrouter is now a CLI:

- src/rules.ts: surface policy as data — family defaults (anthropic/* ->
  messages, openai/* -> responses), surface compat rules, and benchmark-
  proven exceptions with revalidateAfter dates that sync warns about
- src/generate.ts: deterministic whole-catalog generation from the live
  /models API; output depends only on (catalog, rules), never on the
  installed pi version's built-in catalog
- src/sync.ts + src/cli.ts: pi-openrouter sync [--check] [--stdout] [--file]
  rewrites providers.openrouter.models (tool-owned) while preserving
  modelOverrides, provider compat, and other providers; fetch failure
  aborts without touching models.json
- benchmark harness unchanged apart from the shared model builder

No preferred_min_throughput floor: its interaction with allow_fallbacks
under congestion is untested.