Auto-upgrade stale hooks on `partio enable`

[jcleira]

Problem

When a user upgrades the Partio CLI binary, the git hook scripts installed in their repositories become stale — they may reference old binary paths, use outdated shim logic, or miss new hooks added in later versions. Currently, partio enable short-circuits entirely when .partio/ exists (cmd/partio/enable.go:38-42), telling the user "partio is already enabled" without checking whether the installed hooks are up to date.

The only workaround is partio disable && partio enable, which users must know to do and which unnecessarily tears down and rebuilds state.

Desired behavior

partio enable should detect stale hooks and silently upgrade them:

1. If .partio/ exists, skip directory/config creation but still check hooks.
2. For each expected hook (pre-commit, post-commit, pre-push), read the installed script from the hooks directory.
3. Compare the installed content against the expected content from hookScript() / hookScriptAbsolute().
4. If the content differs, overwrite the hook file (preserving the existing backup chain).
5. If a hook is missing entirely, install it following the normal backup logic.
6. Print a summary of what was upgraded.

Context

• cmd/partio/enable.go — the .partio/ existence check that needs to be relaxed
• internal/git/hooks/install.go — installHooks() needs an upgrade-aware code path
• internal/git/hooks/hooks.go — hookScript() / hookScriptAbsolute() generate expected content

Source

Inspired by entireio/cli PR #775 — rewrite hook content when it differs from expected.