Align the setting categories in AGENTS.md

[SaladDay]

Summary

AGENTS.md listed four setting categories, but docs/configuration.md (the owning document) names three: process settings, secrets (including the Core key digest derived from the Core key), and runtime settings in Core's database. AGENTS.md also linked secrets to #installation-directory, a section that only points on to the Compose data volume.

This change makes the AGENTS.md sentence match the owning document:

• It drops "derived files" as a separate category. The only derived secret, the Core key digest, is listed under secrets. Its link pointed at the oac apply steps, which do not define a category.
• It links secrets to #compose-installations, where secrets/ is described.

This is the wording change (D8 in the architecture audit) held back from #513 so that it could be reviewed on its own.

Checks

• The anchors exist in docs/configuration.md.

🤖 Generated with Claude Code

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