OR-231: Safe live file refresh

[sox8502]

Summary

• refresh artifacts from existing file events and worktree files during active sessions, reconnects, idle transitions, and focus
• protect editor drafts with SHA-256 versions, conditional saves, typed conflicts, guarded reloads, and in-memory tab restoration
• finish the OR-231 file-pane UX: 40% default panel, responsive artifact explorer, bounded tree width, and IDE-style file/artifact context menus

Test plan

• pnpm test
• pnpm build
• pnpm lint:styles
• cargo fmt --all --check
• cargo clippy --all-targets -- -D warnings
• cargo build --locked
• cargo test --locked
• live API smoke: checkout version, branch null version, missing-file 409 contract
• externally edit an open clean worktree file and confirm focus/active-agent refresh
• externally edit or delete a dirty file and confirm the draft and conflict actions remain safe
• rewrite/delete/replace an open artifact and confirm its preview refreshes without unrelated media resets
• confirm narrow artifact browsing, pane sizing, and file/artifact context menus in the browser

Tracks OR-231.