docs: fix stale router extension-point description in ARCHITECTURE.md

[shrdgn]

What & why

docs/ARCHITECTURE.md's "Extension points" section and request-flow diagram both referenced router.route() as the entry point server.py calls before gather_panel, and described an LLM classifier as a future extension ("swap in an LLM classifier behind the same RouteDecision return type").

Both are stale:

• server.py doesn't call router.route() — it imports and calls router.route_request() (server.py:495), which internally dispatches to either the classifier or the synchronous heuristic route().
• The LLM classifier isn't a future extension point — it's already shipped as RouterMode.MODEL / router._classify_route, documented in README.md with a full YAML example and covered by tests/test_router.py.

The module table one row above (router.py — "heuristic or model classifier") already got this right; only the "Extension points" prose and the flow diagram were left behind after the classifier shipped. Fixed both references to point at the real entry point and describe the classifier as already supported.

How it was tested

• ruff check . passes (docs-only change, no code touched)
• pytest -q passes (no live network) — no code changed, ran full suite as a sanity check
• New behavior has a test — not applicable, docs-only
• Docs updated if config / request surface / defaults changed — this is the docs fix
• No secrets, prompts, or response bodies added to logs or metrics
• Quality/cost claims backed by a reproducible bench/run.py number — not applicable

Notes for reviewers

Found via an automated repo-review scheduled task. Docs-only, zero code changes.

---

Generated by Claude Code