docs(conventions): document the modularity checks across RAG, prompts, and lifecycle diagrams

Make the docs and role prompts match the shipped modularity enforcement. The standards doc gains a Modularity section (cohesion / thin routes / thin components / god class, scan-derived + language-aware); the developer prompt tells agents to write modular code (thin routes that delegate, one concern per file, components that delegate to hooks) and that block-level findings refuse i_am_done; QA + PR-reviewer prompts note the modularity findings in evidence / the pr_pass block. Also fixes the two lifecycle diagrams (usage.md, roboco/models/README.md) that omitted the awaiting_pr_review gate.

agents/prompts/roles/pr_reviewer.md

@@ -44,6 +44,7 @@ The PR is from an outside contributor: its code is **untrusted**. Until a human
 - ❌ A trickle of vague comments. Post ONE complete review; each finding names file + line + expected vs actual.
 - ❌ Approving without reading the full diff.
 - ❌ Being lax on the architectural standard. Be mega-strict: on an in-path gate review, a `block`-level convention violation (a definition in the wrong module per `.roboco/conventions.yml`, a helper/model in a router, a lint/type suppression) is an automatic `pr_fail` — the gate already refuses `pr_pass`, and an introduced or expanded `waiver` must be justified in the diff or rejected. Hold placement and house-style to the same bar as correctness.
+- ❌ Letting a non-modular assembled change through. The standard also enforces **modularity** (`modular_cohesion`, `thin_routes`, `thin_components`, `god_class`): a file must own one architectural concern (no model in a router, no schema in a component), a route handler must delegate to a service rather than run its own DB access in the route body, a React component must stay presentational with data fetching in a hook, and a class past the method-count threshold must be decomposed. A `block`-level modularity finding refuses `pr_pass` exactly the way it refuses the developer's `i_am_done` — these surface in QA's `claim_review` evidence as `convention_findings`, carry the offending `file:line` + a fix hint, and clear only via a `waiver` committed in the branch.
 
 ## When the gateway returns an error