Docs: document vg doctor checks and CI exit semantics

[vibgrate-team]

Context

vg doctor is the natural “is this install healthy?” entry point for new users and CI images. Public docs should list what it checks, which exits mean “fix your environment,” and how it relates to vg build / vg scan without duplicating the full exit-code matrix already tracked elsewhere (#234).

What to do

1. Document each doctor check category at a high level (runtime, grammars, graph cache, optional modules).
2. Document exit code expectations for CI (pass vs actionable fail).
3. Short vg doctor snippet; local-first; no secrets.

Acceptance

• Newcomer can interpret doctor output and decide next steps
• Cross-link exit-code docs (Docs: document vg scan / vg build exit codes for CI gating #234) rather than forking a second full matrix
• DCO; prefer vg in examples