feat: support coding agent sidebar

[0xranx]

• Add Tauri-side Agent RPC/commands (Codex/Claude/OpenCode) with tool/permission flows, terminal sessions, and AI/Context/Search/Terminal command support.
• Introduce Agent and Terminal sidebars in the UI with multi-session chat, model selection, tool output, permission prompts, and streaming updates.
• Update README and i18n copy to highlight “bring your own coding agent CLI + GUI + built-in Skills/tools,” plus add an overview image.
• Provide default Codex model options in Settings and keep sidebar model selection aligned.

Fixes #16