[git-hooks] Make packaged hooks canonical (#92)

[coisa]

Summary

• Add packaged pre-commit and commit-msg hook templates under resources/git-hooks.
• Update post-checkout and post-merge to refresh dependencies when composer.json or composer.lock changes.
• Export DEVTOOLS_* context variables from post-checkout and post-merge hooks.
• Simplify GitHooksCommand so it only copies/chmods packaged hooks and no longer runs grumphp git:init.
• Update tests and docs to remove the --skip-grumphp-init behavior.

Verification

• rg -n "BASE_DIR|skip-grumphp-init|grumphp git:init|hooks_dir" . --glob '!vendor/**' --glob '!tmp/**'
• sh -n resources/git-hooks/commit-msg && sh -n resources/git-hooks/pre-commit && sh -n resources/git-hooks/post-checkout && sh -n resources/git-hooks/post-merge
• git diff --check
• ./vendor/bin/phpunit --filter GitHooksCommandTest (blocked: env: php: No such file or directory)
• ./vendor/bin/phpunit --filter 'FilesystemTest::symlinkAndReadlinkWillUseAbsolutePaths' (blocked: env: php: No such file or directory)

Notes

• I kept this scoped to the git hook source of truth only. No BASE_DIR, hooks_dir, grumphp.yml, .gitattributes, or update-composer-json changes are included.
• The original stash remains available; I applied it, resolved the relevant hook changes, and left unrelated removed-scope changes out of the PR.

Closes #92

[github-actions]

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