refactor(tauri): split lib.rs into commands/ submodule (Phase 4)

[jamesbrink]

Summary

• Carves the 2,438-line src-tauri/src/lib.rs into a commands/ submodule grouped by concern, mirroring the shell/ submodule split from refactor(tauri): split shell.rs into shell/ submodule (Phase 5) #49 (Phase 5). lib.rs shrinks to 755 lines (-69%) and now holds only the agent supervisor, agent IPC, paste persistence, tracing init, and run().
• All IPC command names preserved — frontend invoke() callers don't need to change.
• Tray idempotency tests travel with their code; lib.rs keeps a smaller test pinning the invoke_handler wiring contract.

Closes #41.

New layout

File	Lines	Concern
lib.rs	755	agent supervisor, agent IPC (start_agent, send_message, agent_command, force_restart_agent, dispatch_a2ui_event), save_paste_image, logging, run()
commands/config.rs	336	read_state, write_state, read_config, write_config, aethon_home_dir, aethon_state_path
commands/session.rs	345	search_sessions, delete_session, export_chat_markdown + snippet helpers
commands/extensions.rs	895	ExtensionMenuItem/Store, set_extension_menu_items, install_app_menu/install_tray/TRAY_ID, AgentWatcher, watch/unwatch_project_extensions, install_aethon_extension
commands/git.rs	152	git_status, pick_project_directory
commands/window.rs	79	toggle_fullscreen, toggle_devtools, updater_available + pubkey gate
commands/mod.rs	23	submodule declarations

Notes

• sanitize_filename_segment moved to helpers.rs since both save_paste_image (lib.rs) and export_chat_markdown (session.rs) need it.
• install_app_menu / install_tray / TRAY_ID bundled into extensions.rs rather than left in lib.rs — the menu rebuild is driven by extension menu deltas, so cohesion lives there. Keeping them in lib.rs would have forced a pub(crate) purely for set_extension_menu_items to reach them.
• Visibility widened on AgentProcess, AgentReloadFlag, agent_reload_in_progress, project_root, resolved_login_path (now pub(crate)) so the watcher debounce worker and install_aethon_extension can reach them from the new module.
• Spec listed search_sessions_by_keyword for the session command; kept the existing IPC name search_sessions to avoid breaking src/skills/default-layout/search-panel.tsx:178 and src/App.tsx:3663.

Test plan

• cargo clippy --all-targets -- -D warnings — clean
• cargo test --lib — 74 passed (3 in the relocated commands::extensions::tests)
• bunx vitest run — 305 passed across 30 files
• bunx tsc -b --noEmit + bunx eslint . — clean
• Full devshell check gate end-to-end — passes
• Smoke test the dev build: send a chat, search sessions, export a chat, install an extension, watch a project's .aethon/extensions/, toggle fullscreen/devtools — all IPC paths resolve from the frontend.