feat(agent): unify models and validate OpenCode providers

Generate chat, task and learning models from agent-models.json.
Use WaterNetwork fast/expert models with 272k context and validate
upstream names before readiness. Add an isolated model check command
with signal-safe cleanup and plugin, Skill and MCP management docs.

Verified in isolation: type checks, 26 focused tests and both model
catalog checks; no model inference requests.

tests/integration/provider-catalog-smoke.ts

@@ -0,0 +1,2 @@
+// Opt-in native catalog verification. Shares the startup check and never infers.
+import "../../scripts/check-models.js";