[ai] Replace guard-commands with block-builds, block-worktrees, and create-worktree...

[ai] Replace guard-commands with block-builds, block-worktrees, and create-worktree hooks.

Splits the monolithic guard-commands hook into three single-purpose
hooks with more granular matchers in settings.json.

block-builds:
  Guards guix and make build/reconfigure commands. Asks on main,
  denies in worktrees for destructive operations (reconfigure,
  home-*, system-*).

block-worktrees:
  Denies git checkout/switch outright. Enforces EnterWorktree
  (must not already be in a worktree) and ExitWorktree (must not
  have unpushed commits diverging from main). Sends a desktop
  notification on EnterWorktree success.

create-worktree:
  Uses a universal matcher ("*") to deny all tool calls on main
  unless already inside the session's worktree. Allows
  EnterWorktree through to let the user enter an isolated session.