Checkout nudge: verify user-defined hook presence

[dpritchett]

Context

snag's post-checkout nudge already nags when snag hooks aren't installed. This issue extends that to user-defined hooks — scripts or tools you want wired into your local checkout that aren't snag builtins.

Use case: "I work across 100+ repos. On the day I clone a repo and every visit until I address it, snag should nag me about missing hooks — both snag builtins and any custom ones I've declared."

Example config

# snag.toml or snag-local.toml

[[hooks]]
phase = "pre-rebase"
run = "~/bin/my-personal-pre-rebase-hook"

[[hooks]]
phase = "prepare-commit-msg"
run = "./scripts/team-commit-template.sh"

Behavior

• snag check checkout verifies these scripts exist and are executable
• If missing or not executable, warn the user (same nudge style as missing snag hooks)
• No execution — snag doesn't run these scripts. Lefthook (or .git/hooks/) handles that. Snag just checks they're in place.
• Resolve ~ to home dir, relative paths from repo root

Out of scope

• Running external scripts (lefthook already does this)
• Remote URLs
• Passing args or managing script output

Prior discussion

Originally proposed as a full external script execution framework. Re-scoped per discussion: snag stays focused on policy enforcement. Verifying hook installation fits that mission; running hooks does not.