docs: populate GEMINI.md with project context

GEMINI.md

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+# Termite - AI Agent Context
+
+## Project Overview
+Terminal app utilities library for Go - provides progress bars, spinners, cursor control, and multi-line matrix layouts.
+
+## Key Components
+| Component | File | Purpose |
+|-----------|------|---------|
+| **Matrix** | `matrix.go` | Multi-row terminal layout for concurrent tasks |
+| **Spinner** | `spinner.go` | Animated progress indicator |
+| **ProgressBar** | `progress_bar.go` | Horizontal progress bar |
+| **Cursor** | `cursor.go` | Terminal cursor control |
+
+## Build & Verify
+```bash
+make format lint test   # Run all checks
+go run ./internal       # Run demo
+```
+
+## Testing Conventions
+- All tests use testify/assert
+- Use `bytes.Buffer` to capture terminal output
+- Tests verify behavior, not internal state
+
+## Code Style
+- Interfaces defined before implementations
+- Formatters allow customization of visual elements
+- Context-based cancellation for all async operations