Rebrand Chorus to Mast

[udaychandra]

Summary

• Rename user-facing Chorus surfaces to Mast, including the app name, CLI command, URL scheme, paths, bundle metadata, install scripts, workflows, and fork docs.
• Replace the previous JetBrains default theme naming with first-party Mast dark/light themes and Mast icon theme defaults, with a darker IntelliJ New UI-inspired palette and compact workspace pane styling.
• Update the SAIL integration and remote-development packaging to call sail, use ~/.sail, and ship mast-remote-server artifacts.

Verification

• jq empty assets/themes/mast/mast-dark.json assets/themes/mast/mast-light.json assets/icon_themes/mast/mast-icons-theme.json
• cargo fmt --all -- --check
• cargo check -p settings_content -p theme_settings -p sing_bridge -p sing_orchestrator -p sing_project -p sing_spec -p remote -p remote_server -p auto_update -p cli -p install_cli -p client -p zed
• cargo test -p theme_settings loads_bundled_mast
• cargo test -p sing_bridge
• cargo test -p client test_parse_zed_link_accepts_primary_scheme
• cargo test -p remote remote_server
• cargo test -p zed test_parse_shared_agent_thread_url
• cargo xtask check-workflows

Release Notes:

• Rebranded Chorus as Mast with Mast defaults, packaging names, URL scheme, and SAIL integration.