docs(indexes): skim layer skimmable, one undescribed entry described

An index is the skim layer: its entries decide whether a reader opens the page, so an entry
that runs 250 characters is a paragraph wearing a bullet. Four entries in `tools/index.md`
(`dotfiles-deploy`, `packages`, `pcie-probe`, `remote-job`) now say what the page is for in one
line and let the page carry the detail — which is the whole argument for progressive disclosure
having an index at all.

`engineering/infra/index.md` pointed at `../sdlc.md` with no description at all; it now carries
the one from that page's frontmatter, like its four siblings.

Refs #64

omp-docs/engineering/infra/index.md

@@ -12,4 +12,4 @@ Current (2025/2026) best practices for infrastructure as code, containers, orche
 ## See Also
 
 * [Engineering](../index.md) - The parent area.
-* [SDLC and Tooling](../sdlc.md)
+* [SDLC and Tooling](../sdlc.md) - git, CI/CD, testing, review, versioning, and dev environments.