Problem

mobilecli spawns tmux with -f /dev/null (or NUL on Windows), which completely ignores the user's ~/.tmux.conf. This causes two pain points:

1. Warnings in modern terminals — applications using the kitty keyboard protocol or other extended key sequences emit "extended-keys is off" warnings because tmux hasn't been told to enable them.
2. No escape hatch for power users — users who have customized tmux configs (colors, key bindings, etc.) get a barebones tmux environment inside mobilecli sessions.

Solution

This PR takes the "careful best of both worlds" approach:

1. Surgical fix: enable extended-keys by default

setup_tmux_session() now explicitly runs set-option -g extended-keys on after setting up the session. This silences warnings without giving up control over tmux behavior.

2. Opt-in custom tmux config

Instead of broadly loading ~/.tmux.conf (which would break mobilecli's streaming assumptions), we introduce a two-tier lookup:

When a custom config is provided, mobilecli still loads it, then immediately enforces its own required options:

• history-limit 200000
• mouse (per platform policy)
• window-size latest
• status off, allow-rename off
• extended-keys on

This means user configs only affect things mobilecli doesn't care about (colors, key bindings, etc.), while streaming reliability is preserved.

3. Cross-platform cleanup

• Extracted tmux_null_device() helper to centralize the Windows NUL vs Unix /dev/null logic.
• Both the std::process::Command setup path and the portable_pty::CommandBuilder attach path now use the same config resolution.

4. Safety hardening

• resolve_tmux_config_path() uses path.is_file() instead of path.exists() to reject directories (defensive against misconfigured env vars).
• Refactored tracing::info! block to use if let instead of is_some() + unwrap() (clippy clean).

Testing

• cargo test pty_wrapper passes (10/10)
• cargo clippy passes cleanly
• Added new test: tmux_session_enables_extended_keys_by_default
• Updated existing tests to pass explicit None for config path

Documentation

• Added tmux.conf to the ~/.mobilecli/ file table in README
• Documented MOBILECLI_TMUX_CONFIG, enforced options, and warnings about dangerous tmux settings (destroy-unattached, remain-on-exit)
• Documented that daemon-spawned headless sessions use the default tmux server (different from pty-wrapper attach sessions)

Why not just load ~/.tmux.conf automatically?

User tmux configs can override behavior mobilecli depends on:

• mouse on breaking desktop clipboard selection
• alternate-screen settings interfering with scrollback capture
• status on wasting vertical space on mobile screens
• Custom destroy-unattached or remain-on-exit breaking session lifecycle

The opt-in model keeps the safe default for 99% of users while giving power users an escape hatch.

Known limitations

• Daemon-spawned headless sessions (phone-only, no desktop terminal) currently use the default tmux server and load ~/.tmux.conf, not ~/.mobilecli/tmux.conf. This is intentional because headless sessions don't need the isolated control that attach-mode sessions require. If there's demand, we can extend custom config support to headless sessions in a follow-up.
• On Windows, tmux is typically installed via MSYS2/Git Bash. Backslash path separators in MOBILECLI_TMUX_CONFIG are passed through as-is; most Windows tmux builds accept forward slashes, so users with custom configs may want to use those.

Closes the "extended-keys is off" warning issue and provides a path for tmux customization without compromising streaming reliability.