Add diff viewer line marking and submitting fixes by line

AGENTS.md

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+# PR Dojo — Agent Guide
+
+## Project
+**Code Review Hunter** ("PR Dojo") — an Astro site where users practice finding bugs in rejected PR code, earn XP, and submit fixes. Currently a static Astro frontend; backend (Quarkus or Node.js+SQLite) is planned per `full_plan.md`.
+
+## Developer Commands
+```
+bun install          # install deps
+bun dev              # start dev server at localhost:4321
+bun build            # static build → dist/
+bun preview          # preview the build locally
+bun astro ...        # run Astro CLI (add, check, etc.)
+```
+**Use `bun`, not npm or npx.**
+
+## Tech Stack
+- **Astro 6** with **Tailwind CSS v4** via `@tailwindcss/vite` plugin (not the PostCSS pipeline)
+- **TypeScript** strict mode via `astro/tsconfigs/strict`
+- **Node >= 22.12.0** (enforced in `package.json` engines)
+- No test framework, linter, or formatter configured yet
+
+## Architecture
+```
+src/
+  data/challenges.json     # static challenge data (id, code, hints, expectedLines, expectedPatch)
+  pages/
+    index.astro            # challenge listing + hero
+    profile.astro          # user profile / XP dashboard
+    challenges/[slug].astro # single challenge view with DiffViewer
+  components/
+    DiffViewer.astro       # side-by-side code diff UI
+    Welcome.astro          # legacy starter component
+  layouts/Layout.astro     # root layout wrapping header/nav/footer
+  styles/global.css        # global styles
+```
+
+## Challenge Data Schema
+Each entry in `src/data/challenges.json`:
+- `id`, `title`, `repository`, `baseSha`, `difficulty` (1-5), `xpValue`, `bugType`
+- `file`, `code` (buggy source), `hints` (array of "Line N: description")
+- `expectedLines` (array of line numbers), `expectedPatch` (unified diff string)
+
+## Key Constraints
+- Tailwind v4 uses the Vite plugin, not PostCSS. Do not add `tailwind.config.js` or `@layer` directives — they won't work.
+- Astro SSR renders everything client-side by default. Add `client:*` directives only when interactivity is needed.
+- `full_plan.md` documents the planned backend architecture and MVP roadmap. It is a reference, not a spec to implement blindly.
+- No CI, no pre-commit hooks, no test runner. If you add tests, document the command in this file.