pre-commit: incorrect executable bits (6 .md marked +x; 2 .py scripts missing +x)

Part of #213.

## Problem

Two complementary pre-commit hooks fail because of wrong Unix executable bits:

### `check-executables-have-shebangs` — Failed
These `.md` files are marked executable (`100755`) but have no shebang. Docs should not be executable:

```
plugins/sagemaker-ai/skills/hyperpod-issue-report/SKILL.md
plugins/sagemaker-ai/skills/hyperpod-issue-report/references/collection-details.md
plugins/sagemaker-ai/skills/hyperpod-issue-report/references/troubleshooting.md
plugins/sagemaker-ai/skills/hyperpod-ssm/SKILL.md
plugins/sagemaker-ai/skills/hyperpod-ssm/references/troubleshooting.md
plugins/sagemaker-ai/skills/hyperpod-version-checker/SKILL.md
```

Fix: `git update-index --chmod=-x ` (or `chmod -x`).

### `check-shebang-scripts-are-executable` — Failed
These `.py` files have a shebang but are **not** marked executable:

```
tools/generate_codex_manifests.py
plugins/sagemaker-ai/skills/dataset-transformation/scripts/transformation_tools.py
```

Fix: `git update-index --chmod=+x ` (or remove the shebang if not meant to be run directly).

## Reproduce

```bash
mise exec -- pre-commit run check-executables-have-shebangs check-shebang-scripts-are-executable --all-files
```

_{Filed with assistance from Claude Code while implementing #211.}

Start by running `mise exec -- pre-commit run check-executables-have-shebangs check-shebang-scripts-are-executable --all-files` to confirm the failures. Update the executable bits for the six listed Markdown files and two listed Python scripts, then rerun both hooks with `--all-files`; done means both checks pass.

Written by the indexing model from the issue text.