add sidebar project filter toggle

[joshuavial]

Summary

• Adds a project filter to the sidebar, toggled with p key or workmux sidebar filter [none|project] CLI command
• When active, filters the sidebar to only show agents belonging to the same project as the current window
• Navigation hotkeys (alt-j/k, jump) respect the filter
• Project is detected from the host agent's path, falling back to the host pane's working directory for non-agent windows
• Filter state persists via tmux global and settings.json

Test plan

• just check passes (format, clippy, build, 917 tests, ruff, pyright)
• Manual: press p in sidebar to toggle filter, verify agents are filtered by project
• Manual: workmux sidebar filter project / workmux sidebar filter none from CLI
• Manual: alt-j/k only cycles through filtered agents when filter is active
• Manual: non-agent windows (plain zsh) correctly detect project from cwd