Add cwd and env parameters to Flow.spawn()

[recursiveAF]

Summary

• Add cwd parameter to set working directory for terminal sessions (uses tmux -c flag)
• Add env parameter to set environment variables in the session
• User env vars are merged with mitmproxy env vars (proxy settings take precedence)

Usage

# Before (shell workaround)
Flow.spawn(f"cd {path} && {cmd}")

# After (clean API)
Flow.spawn(cmd, cwd=path, env={"HOME": "/tmp", "GT_TEST_MODE": "1"})

Test plan

• Test cwd sets the working directory correctly
• Test env sets environment variables correctly
• Test cwd and env work together
• Test env vars don't leak between Flow instances

Run tests with: pytest tests/test_flow_cwd_env.py -v -s

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