docs: add README with setup and configuration guide

README.md

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+# codereview
+
+LLM-powered pull request code review as a GitHub Action. Posts inline review comments using [conventional comments](https://conventionalcomments.org/) format and submits a verdict (approve, request changes, or comment).
+
+## Features
+
+- Reviews PR diffs with full file context
+- Uses conventional comment labels: `nit:`, `suggestion:`, `issue:`, `question:`, `thought:`, `chore:`, `praise:`
+- Replies to comment threads when developers respond
+- Resolves threads automatically when concerns are addressed
+- Remembers prior comments to avoid repeating itself
+- Configurable LLM provider (Gemini supported, more planned)
+
+## Quick start
+
+Add a workflow file to your repository:
+
+```yaml
+# .github/workflows/review.yml
+name: Code Review
+
+on:
+  pull_request:
+    types: [opened, synchronize]
+  pull_request_review_comment:
+    types: [created]
+
+permissions:
+  pull-requests: write
+  contents: read
+
+jobs:
+  review:
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: LimpidTech/codereview@main
+        with:
+          gemini-api-key: ${{ secrets.GEMINI_API_KEY }}
+```
+
+Get a Gemini API key from [Google AI Studio](https://aistudio.google.com/apikey) and add it as a repository or organization secret named `GEMINI_API_KEY`.
+
+## Inputs
+
+| Input | Required | Default | Description |
+|-------|----------|---------|-------------|
+| `github-token` | no | `${{ github.token }}` | GitHub token for API access |
+| `provider` | no | `gemini` | LLM provider to use |
+| `gemini-api-key` | no | | API key for Google Gemini |
+| `model` | no | `gemini-2.5-flash` | Model name override |
+| `instructions` | no | | Additional review instructions |
+| `bot-login` | no | `github-actions[bot]` | Bot username for loop prevention |
+
+## Providers
+
+### Gemini (default)
+
+```yaml
+- uses: LimpidTech/codereview@main
+  with:
+    provider: gemini
+    gemini-api-key: ${{ secrets.GEMINI_API_KEY }}
+```
+
+To use a specific model:
+
+```yaml
+- uses: LimpidTech/codereview@main
+  with:
+    gemini-api-key: ${{ secrets.GEMINI_API_KEY }}
+    model: gemini-2.5-pro
+```
+
+### Adding providers
+
+The codebase is designed for easy provider addition. Each provider implements a single function type:
+
+```go
+type ReviewFunc func(ctx context.Context, req Request) (Response, error)
+```
+
+See `internal/provider/gemini/` for a reference implementation.
+
+## Custom instructions
+
+Use the `instructions` input to guide the reviewer toward your project's conventions:
+
+```yaml
+- uses: LimpidTech/codereview@main
+  with:
+    gemini-api-key: ${{ secrets.GEMINI_API_KEY }}
+    instructions: |
+      This is a Go project using the standard library only.
+      We prefer table-driven tests.
+      Error messages should not be capitalized.
+      All exported functions must have doc comments.
+```
+
+Instructions are prepended to the review prompt, so the LLM will consider them alongside the diff and file context.
+
+## How it works
+
+### On PR open / push
+
+1. Fetches the PR diff and full contents of changed files
+2. Fetches any prior review comments it has already made
+3. Sends everything to the configured LLM with review instructions
+4. Posts inline review comments with conventional labels
+5. Submits a verdict: `APPROVE`, `REQUEST_CHANGES`, or `COMMENT`
+
+### On review comment reply
+
+1. Fetches the full conversation thread
+2. Sends the thread context and relevant diff hunk to the LLM
+3. Posts a reply
+4. If the concern is resolved, automatically resolves the thread (only for threads it started)
+
+## Permissions
+
+The workflow needs these permissions:
+
+```yaml
+permissions:
+  pull-requests: write  # post reviews, reply to comments, resolve threads
+  contents: read        # fetch file contents for context
+```
+
+## Using with a GitHub App token
+
+If you use a GitHub App token instead of the default `GITHUB_TOKEN`, set `bot-login` to the app's bot username so it doesn't reply to its own comments:
+
+```yaml
+- uses: LimpidTech/codereview@main
+  with:
+    github-token: ${{ steps.app-token.outputs.token }}
+    gemini-api-key: ${{ secrets.GEMINI_API_KEY }}
+    bot-login: my-app[bot]
+```
+
+## License
+
+BSD 2-Clause. See [LICENSE](LICENSE) for details.