fix(server): restrict HTTP file read/list to allowed paths

[y4sol]

Summary

Restricts filesystem access through GET /api/file/content and GET /api/file/list by validating paths only in the HTTP routes. Internal File.read() callers (e.g. memory under user data directories) are unchanged.

Problem

Clients that could reach the API could read arbitrary local files the server process could open.

Solution

• Add flocks/utils/http_file_read_guard.py with resolve_path_for_http_file_access():
  • Allowed roots: Flocks project directory (ancestor containing .flocks/, without falling back to cwd as a project root when none exists), configured data directory, workspace directory, optional allowReadPaths entries (validated), and a small built-in allowlist for common safe host files (e.g. /etc/hosts when they exist).
  • Denied: Flocks config tree, ~/.ssh, and on POSIX /proc, /sys, /dev.
  • Uses assert_sandbox_path for containment and symlink checks.
• Wire the guard into flocks/server/routes/file.py for /content and /list; PermissionError → HTTP 403.
• Add ConfigInfo.allow_read_paths with Pydantic alias allowReadPaths for JSON config.
• Add find_flocks_project_root() (returns None if no .flocks ancestor); find_project_root() keeps backward-compatible behavior for non-security callers.

Configuration

Optional extra paths in user config, e.g. ~/.flocks/config/flocks.json:

{
  "allowReadPaths": ["/opt/myapp/config"]
}