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

models.ts

@@ -173,6 +173,7 @@ export const providers = {
         displayName: "GPT Codex Mini",
         resolve: "openrouter/openai/gpt-5.1-codex-mini",
       },
+      "o4-mini": { displayName: "O4 Mini", resolve: "openrouter/openai/o4-mini" },
       "gemini-pro": {
         displayName: "Gemini Pro",
         resolve: "openrouter/google/gemini-3.1-pro-preview",