docs(vdb): present `vdb` as primary interface in README + CLAUDE.md

Step 6, first slice. Builds on !12 (merged) (vdb now installs on PATH).

Repoints the top-level docs at vdb while keeping the standalone engines documented (deployed-copy + systemd still supported):

• README quickstart drives vdb context add → vdb index → vdb serve, with a "no install?" fallback for python3 vault-indexer.py --vault-root …; MCP registration (Claude Code, OpenCode) uses vdb serve --vault …; models/agents/fixtures snippets use the vdb subcommands; Files table gains vdb/ rows and drops the stale "vdb-menu = scaffold" line.
• CLAUDE.md commands block becomes vdb … with standalone equivalents inline; new "Two ways to point the engines at a vault" section (runtime resolution vs. hardcoded-constant deployed-copy); apply_config note points at vdb/vdb_config.py.

Docs-only; tool count (28) unchanged. Remaining ref sweep (harness docs, skill SKILL.md files, docs/setup-guide.md, AGENTS.md, tests/tui/README) is a tracked follow-up.

🤖 Generated with Claude Code