restructure docs: split triggers into usage pages, add model resolution docs

- split triggers.mdx into direct-prompting, pr-reviews, issue-enrichment, coding-tasks
- rename manual-setup.mdx to headless-action.mdx (CI integration)
- reorganize sidebar into Getting started / Usage / Reference groups
- add redirects for /triggers and /manual-setup
- add PULLFROG_MODEL env var support across action, workflows, and docs
- rewrite models.mdx with aliases, free models, resolution chain, routers
- update all cross-references in app, components, and docs

Made-with: Cursor

test/ci.test.ts

@@ -63,6 +63,7 @@ const dynamicAgentsExpression = "$" + "{{ fromJSON(needs.changes.outputs.agents)
 const expectedAgentEnvVars = [
   "GITHUB_TOKEN",
   ...new Set(Object.values(providers).flatMap((p) => [...p.envVars])),
+  "PULLFROG_MODEL",
   "OPENCODE_MODEL",
 ].sort();