[hooks] pre-flight checks, surface failures, prettify

[hooks] pre-flight checks, surface failures, prettify

Every hook now checks that GUIX_BIN and required commands exist
before running.  PreToolUse hooks deny with a visible reason on
failure; other hooks emit systemMessage.  All hooks follow the
create-worktree style: copyright header, individual check blocks.

Fixed block-worktrees to not silently exit when CLAUDE_PROJECT_DIR
is unset (the hook derives proj_root from git, not that variable).

Fixed git push exemption in create-worktree to match full paths
like $HOME/.guix-profile/bin/git push.

Co-Authored-By: Claude <noreply@anthropic.com>
hooks: guix-build check for divergence from main

Exit early only when the tree is clean AND at main tip.  Warn and
stop if the branch is behind main (needs rebase).  Fall through to
queue logic when there are staged commits or a dirty tree.

Co-Authored-By: Claude <noreply@anthropic.com>