Generated AGENTS.md links to missing static/llms.txt

### Summary

The generated `docs/AGENTS.md` currently includes this entry:

```md
- [Static](./static/llms.txt)
```

However, `docs/static/llms.txt` does not exist. The repository has a `docs/static/` directory, but it appears to contain static assets rather than a documentation section with its own generated `llms.txt` file.

### Where it seems to come from

`docs/AGENTS.md` is generated by `scripts/agents.js`, which calls `discoverTopLevelSections()` from `scripts/lib/docs-utils.js`.

`discoverTopLevelSections()` includes top-level directories unless they are listed in `CONSTANTS.skipDirs`. `static` is not currently in that skip list, so the generator treats `docs/static/` as a docs section and emits a link to `./static/llms.txt`.

### Expected behavior

The generated AGENTS index should not link to `./static/llms.txt` unless that file is generated/committed. If `docs/static/` is only an asset directory, it should likely be skipped by the section discovery logic.

### Reproduction

1. Inspect `docs/AGENTS.md`.
2. Find the `Static` entry under the LLM entry point section.
3. Check that `docs/static/llms.txt` is missing.

### Possible fix

Add `static` to the docs generator skip list, or otherwise exclude asset-only directories from `discoverTopLevelSections()`.

Start with scripts/agents.js and scripts/lib/docs-utils.js, then inspect docs/AGENTS.md and the docs/static/ directory. Trace how discoverTopLevelSections() uses CONSTANTS.skipDirs and verify the generated index no longer links to ./static/llms.txt. Done means asset-only directories are excluded unless their generated llms.txt exists.

Written by the indexing model from the issue text.