feat: add API key color assignment, auto-persist, AGENTS.md

AGENTS.md

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+# AGENTS.md
+
+## Project Overview
+- **Primary Language**: C# (.NET 8 Console Application)
+- **Key Libraries**: RestSharp, Newtonsoft.Json, System.Data.SQLite, Microsoft.Extensions.Configuration
+- **Purpose**: Tumblr API data harvester for collecting notes, posts, likes, and replies, storing results in SQLite.
+
+## Architectural Patterns
+- CLI entry point in `Program.cs` with workflow orchestration
+- `DataAccess.cs`: Database operations, `ApiKeyPool` (API key management), `APIAccess` (Tumblr client)
+- `ResponseNotes.cs`: Tumblr API response models
+- Round-robin API key rotation with rate-limit tracking
+- Automatic console color assignment per API key for output differentiation
+
+## Developer Guidelines
+
+### Code Formatting
+- 4-space indentation, no tabs, match existing C# style
+- PascalCase for public members, camelCase for locals
+- Minimize code comments unless explicitly requested
+- Use only existing project libraries; no new dependencies without confirmation
+- Match accessibility modifiers (`public` for models, `internal` for helpers)
+
+### Error Handling
+- Wrap file/network operations in `try-catch`
+- Log non-critical errors (e.g., config write failures) with `[Warning]` prefix
+- Preserve console color state: use save/restore pattern for temporary color changes
+- API rate limits must use `ApiKeyPool.MarkRateLimited()`/`MarkAvailable()`
+
+### Testing
+- No existing test suite; use xUnit if adding tests
+- Test critical logic: `ApiKeyPool` init, color parsing, config persistence
+- Avoid testing one-off CLI workflows
+
+### Git Commit Messages
+- Imperative mood ("Add feature" not "Added feature")
+- Prefix with type: `feat:`, `fix:`, `chore:`, `docs:`
+- Keep messages under 72 characters
+- Never commit sensitive data (API keys/tokens)
+
+### API Key Color Rules
+- Unconfigured keys auto-assign colors from a preset palette
+- Auto-assigned colors persist to `appsettings.json`
+- All output for an active key uses its assigned color
+- Temporary color changes (e.g., errors) must restore the key's color afterward