docs(vdb): repoint skills + setup-guide at vdb; add config plan/detect-viewers

Finishes the step-6 reference sweep. Builds on !14 (merged).

Skill workflows (vdb-setup, vdb-update, vdb-optimizer, vdb-po) + docs/setup-guide.md / AGENTS.md now invoke engines/tooling through vdb, and the menu-flag forms become vdb config <verb>:

vdb-menu.py --list-features         -> vdb config list
--get/--set/--enable/--disable      -> vdb config get/set/enable/disable
--print-config/--plan               -> vdb config print/plan
--migrate/--apply/--detect-viewers  -> vdb config migrate/apply/detect-viewers
bare vdb-menu.py                    -> vdb menu

To keep that mapping complete, vdb/cli.py gains the two missing verbs — vdb config plan and vdb config detect-viewers — delegating to the existing argparse surface. The deployed-copy / systemd note is preserved next to vdb config apply (runtime resolution makes it optional on the installed path). Genuine source-file path references are left as paths.

tests/test_vdb_cli.py covers the new verbs. Full suite green. Completes the unified-vdb-CLI effort (docs/vdb-cli-plan.md).

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