Complete Phase 6: documentation cleanup and integration (#97)

## Summary
- Delete `docs/zk/` directory - all useful content migrated to structured docs
- Delete `docs/README.md` - `docs/index.md` is now the documentation root
- Add `devpi` reference card and `use-pypi-proxy` how-to guide
- Add maintenance notes to `indri` reference (sleep prevention, passwordless sudo)
- Add iCloud Photos backup note to `borgmatic` reference
- Rewrite `zk-docs` mise task to prime AI context with key docs instead of legacy cards
- Update `CLAUDE.md` and `README.md` to remove zk references
- Update `exploring-the-docs` with AI context priming section

This completes the Diataxis documentation restructuring. All six phases are now done.

## Deployment and Testing
- [x] Pre-commit hooks pass (including doc-links validator)
- [ ] Build and deploy to docs.ops.eblu.me to verify rendering

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/97

docs/reference/services/borgmatic.md

@@ -25,7 +25,9 @@ Daily backup system using Borg backup, running on indri.
 - `/opt/homebrew/var/forgejo` - Git forge data
 - `~/.config/borgmatic` - Borgmatic config
 - `~/Documents` - Personal documents
-- `~/Pictures` - Photos
+- `~/Pictures` - Photos (see note below)
+
+**iCloud Photos note:** macOS Photos.app defaults to "Optimize Mac Storage" which keeps only thumbnails locally. Borgmatic only backs up what's on disk, so iCloud-only photos are NOT backed up via this method.
 
 **Databases:**
 - `miniflux` on [[postgresql]]