files: add a safe project and worktree explorer, search, and editor

[guicybercode]

Context

Users need to inspect specifications and agent changes without leaving CLI Master, and canvas File Tree nodes require a safe backend. A generic arbitrary-filesystem API would be too broad.

Goal

Add project/worktree-scoped file browsing, text search, reading, and editing through typed daemon operations.

Scope

• Browse registered project roots and managed worktrees.
• Search filenames and text with bounded results and cancellation.
• Read UTF-8 text and report binary/unsupported encodings safely.
• Write files atomically with revision or content-hash conflict checks.
• Watch scoped filesystem changes and refresh affected views.
• Integrate file selection with the prompt composer and Git status.
• Provide accessible tree/list and editor interactions.

Acceptance criteria

• Requests identify a registered project/worktree plus a validated relative path, never an arbitrary absolute target.
• Path traversal, symlink escape, NUL bytes, and option-like path confusion fail closed.
• Large files and result sets have explicit limits and truncation metadata.
• A stale editor cannot silently overwrite newer disk content.
• Atomic save preserves the original file when a write fails.
• Binary files are never decoded lossily into the editor.
• Search is cancellable and cannot block PTY/event delivery.
• File changes immediately feed the task's Git status view.
• Tests use real trees with Unicode, symlinks, binary files, races, and permission failures.

Out of scope

A full language server, remote filesystem, and arbitrary home-directory browsing.