Add AGENTS.md with codebase documentation for AI agents

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

AGENTS.md

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+# Agent Guidelines for Flickr Mail
+
+This document provides context for AI agents working on this codebase.
+
+## Project Overview
+
+Flickr Mail is a Flask web application that helps users find photos on Flickr
+for Wikipedia articles and contact photographers to request Creative Commons
+licensing.
+
+## Architecture
+
+- **main.py**: Single-file Flask application containing all routes and logic
+- **templates/**: Jinja2 templates using Bootstrap 5 for styling
+  - `base.html`: Base template with Bootstrap CSS/JS
+  - `combined.html`: Main UI template for search, results, and message composition
+  - `message.jinja`: Template for the permission request message body
+  - `show_error.html`: Error display template
+
+## Key Components
+
+### Flickr Search (`search_flickr`, `parse_flickr_search_results`)
+
+Searches Flickr by scraping the search results page. The page embeds JSON data
+in a `modelExport` JavaScript variable which contains photo metadata.
+
+- Uses browser-like headers (`BROWSER_HEADERS`) to avoid blocks
+- Parses embedded JSON by counting braces (not regex) to handle nested structures
+- Returns `FlickrPhoto` dataclass instances with id, title, username, license, URLs
+
+### FlickrPhoto Dataclass
+
+Represents a photo with:
+- `id`, `title`, `path_alias`, `owner_nsid`, `username`, `realname`
+- `license` (int): Flickr license code (0=ARR, 4=CC BY, 5=CC BY-SA, etc.)
+- `thumb_url`, `medium_url`: Static image URLs
+- `flickr_url` property: URL to photo page
+- `license_name` property: Human-readable license name
+
+### License Codes
+
+Wikipedia-compatible licenses (can be used): 4 (CC BY), 5 (CC BY-SA), 7 (No
+known copyright), 8 (US Government), 9 (CC0), 10 (Public Domain).
+
+Not compatible: 0 (All Rights Reserved), 1-3 (NC variants), 6 (ND).
+
+### URL Validation (`is_valid_flickr_image_url`)
+
+Validates that image URLs passed via query params are from legitimate Flickr
+static image servers:
+- `live.staticflickr.com`
+- `farm*.staticflickr.com`
+- `c1.staticflickr.com`, `c2.staticflickr.com`
+
+### NSID Lookup (`flickr_usrename_to_nsid`)
+
+Converts a Flickr username/path alias to the NSID (internal user ID) needed
+for the Flickr mail URL. Scrapes the user's profile page for embedded params.
+
+## Request Flow
+
+1. User enters Wikipedia article title/URL → `start()` extracts article name
+2. `search_flickr()` fetches and parses Flickr search results
+3. Results displayed as clickable photo grid with license badges
+4. User clicks photo → page reloads with `flickr` and `img` params
+5. `flickr_usrename_to_nsid()` looks up the photographer's NSID
+6. Message template rendered with photo details
+7. User copies message and clicks link to Flickr's mail compose page
+
+## Testing Changes
+
+Run the Flask app locally:
+```bash
+python3 main.py
+```
+Then visit http://localhost:5000/
+
+Test search functionality:
+```python
+from main import search_flickr
+photos = search_flickr("Big Ben")
+print(len(photos), photos[0].title, photos[0].license_name)
+```
+
+## Potential Improvements
+
+- Add pagination for search results (currently shows ~25 photos)
+- Cache search results to reduce Flickr requests
+- Add filtering by license type
+- Handle Flickr rate limiting/blocks more gracefully
+- Add tests for the parsing logic