Switch git hooks from pre-commit to prek (#276)

## Summary

- Replace pre-commit with [prek](https://github.com/j178/prek), a faster Rust-native drop-in alternative
- Migrate config from `.pre-commit-config.yaml` (YAML) to `prek.toml` (TOML)
- Add new built-in checks: case conflicts, private key detection, executable shebangs
- Install prek via mise native registry (`aqua:j178/prek`) instead of pipx
- Update all doc references across README, contributing guide, and how-to docs

## Notes

- `check-yaml` still uses the remote `pre-commit-hooks` repo because prek's builtin fast path doesn't support `--unsafe` yet (needed for Ansible custom YAML tags)
- All existing custom hooks (docs validation, container version check, mikado invariant, workflow validation) work unchanged
- Tested: all hooks pass on clean tree, deliberate doc link breakage is caught

## Test plan

- [x] `prek run --all-files` passes all checks
- [x] Broken wiki-link correctly caught by `docs-check-links`
- [x] taplo-format auto-fixes TOML formatting on commit
- [x] commit-msg hook (mikado invariant) fires correctly

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/276

mise.toml

@@ -1,5 +1,5 @@
 [tools]
 "pipx:ansible-core" = { version = "latest", uvx = "true", uvx_args = "--with botocore --with boto3" }
-"pipx:pre-commit" = { version = "latest", uvx = "true" }
+prek = "latest"
 pulumi = "latest"
 dagger = "latest"