feat(auth): add TWITTER_BROWSER env var to specify browser preference

Add TWITTER_BROWSER environment variable to allow users to control
which browser's cookies are prioritized during extraction.

Example: TWITTER_BROWSER=chrome twitter feed

Supported values: arc, chrome, edge, firefox, brave.
The specified browser is moved to the front of the extraction order.

Also adds AGENTS.md developer guide for AI coding assistants.

Co-authored-by: Agassi <413855+agassiyzh@users.noreply.github.com>

AGENTS.md

@@ -0,0 +1,70 @@
+# AGENTS.md — Agent Developer Guide for twitter-cli
+
+This file provides context for AI agents working in this repository.
+
+## Project Overview
+
+- **Project**: twitter-cli — A CLI for Twitter/X (read timelines, bookmarks, search, post, reply, etc.)
+- **Language**: Python 3.10+
+- **Package Manager**: uv (recommended) / pip
+- **Repository**: https://github.com/jackwener/twitter-cli
+
+## Build, Lint, and Test Commands
+
+```bash
+# Install all dependencies (including dev)
+uv sync --extra dev
+
+# Run ruff linter
+uv run ruff check .
+
+# Run mypy type checker
+uv run mypy twitter_cli
+
+# Run all tests (excludes smoke tests by default)
+uv run pytest -q
+
+# Run a single test
+uv run pytest tests/test_cli.py::test_feed_command -v
+
+# Run tests matching pattern
+uv run pytest -k "test_parse" -v
+```
+
+## Code Style
+
+- **Line length**: 100 characters
+- **Python version**: 3.10+
+- Use `from __future__ import annotations` at top of all .py files
+- **Functions/variables**: `snake_case`, **Classes**: `PascalCase`, **Constants**: `UPPER_SNAKE_CASE`
+- Private functions: prefix with `_`
+- Use `@dataclass` for data models (in `models.py`)
+- Use Click framework for CLI commands
+- Custom exceptions in `exceptions.py`, base: `TwitterError(RuntimeError)`
+
+## Project Structure
+
+```
+twitter_cli/
+├── cli.py               # Click CLI entry point
+├── client.py            # Twitter API client (HTTP)
+├── auth.py              # Cookie extraction & auth
+├── graphql.py           # GraphQL query IDs
+├── parser.py            # Tweet/User parsing
+├── models.py            # Dataclass models
+├── formatter.py         # Rich table formatting
+├── serialization.py     # YAML/JSON output
+├── output.py            # Structured output helpers
+├── config.py            # Config loading
+├── filter.py            # Tweet ranking/scoring
+├── constants.py         # Constants
+├── exceptions.py        # Custom exceptions
+├── cache.py             # Tweet caching
+├── search.py            # Search utilities
+└── timeutil.py          # Time utilities
+```
+
+## CI
+
+- GitHub Actions: Python 3.10, 3.11, 3.12
+- CI validates: ruff check + mypy + pytest