Fix: prevent hook reinstall loop when luca install runs inside a git worktree

[albertodebortoli]

Problem

When luca install is run from within a git worktree, the post-checkout hook triggers a failure cascade:

How git worktrees work

In a normal repository, .git is a directory. When you use git worktree add, each worktree gets a .git that is a regular file containing a pointer like gitdir: /path/to/main/repo/.git/worktrees/<name>. The actual hooks live only in the main repo's .git/hooks/ and are shared across all worktrees.

The failure cascade

1. A developer switches branches in a worktree — git fires the post-checkout hook from the main repo's .git/hooks/post-checkout.
2. The hook runs luca install --quiet, which (without this fix) attempts to re-install the post-checkout hook.
3. luca install calls GitHookInstaller.installPostCheckoutHook(), which tries to write to .git/hooks/post-checkout — but in a worktree .git is a file, not a directory, so the path .git/hooks/post-checkout is invalid and the write fails.
4. luca install exits non-zero, the hook logs a warning, and tool synchronisation may be skipped or incomplete.

This happens on every branch switch in every worktree, making the hook unreliable for anyone using git worktree.

Root Cause

The post-checkout hook calls luca install unconditionally without telling it to skip hook (re-)installation. Since the hook is already installed (it's the thing running right now), trying to install it again on every checkout is both unnecessary and broken in worktrees.

Fix

Pass --no-install-post-checkout-git-hook to the luca install invocation in the post-checkout script. This tells the CLI not to attempt hook installation during a hook-triggered run — the hook is already in place.

# Before
luca install --quiet

# After
luca install --quiet --no-install-post-checkout-git-hook

Note: A companion fix LucaTools/Luca#57 is required in the Luca CLI itself (GitHookInstaller.swift) to:

1. Support the --no-install-post-checkout-git-hook flag.
2. Independently guard against worktrees by checking that .git is a directory (not just that it exists) before attempting hook installation.
   This PR addresses the script side; the CLI side should be handled in the luca repository.

Test Plan

• Existing bats test luca install: called with correct args on happy path updated to assert the new flag is passed — all 13 tests in post_checkout.bats pass.
• Manually switch branches in a git worktree and confirm no hook-installation warning appears.
• Confirm tool synchronisation still works correctly in a normal (non-worktree) checkout.

[albertodebortoli]

Accidentally closed by LucaTools/Luca#57.
Proceeding with the merge.