Wizard-side support for discoverable specialist audits. The audit
runner now ships an `--areas` flag, exposes the constraint to the
agent at runtime, and surfaces five new placeholder slides for
discoverable specialists.

- New `--areas="web analytics, feature flags"` CLI flag constrains
  the audit to a subset of the canonical 8-area enum
  (Installation, Identification, Event Capture, Web Analytics,
  Feature Flags, Experiments, LLM Analytics, Error Tracking).
  Unknown values log a hint listing valid areas and continue.
- New `mcp__wizard-tools__audit_get_areas` MCP tool returns
  `{ areas, allowed, constrained }` so the SKILL can honor the
  filter at runtime.
- AuditAreaPane state machine grows two intermediate states for
  the gap between basic specialists finishing and the discovery /
  dispatch agent enrolling discoverable specialists via
  `audit_add_checks`: "Essentials checked" (no enrollments yet)
  and "Running expert subagents" (with the live area list).
- Five placeholder slides — Web Analytics, Feature Flags,
  Experiments, LLM Analytics, Error Tracking — with intro copy and
  docs URLs.
- Specialist registry split: `AUDIT_SPECIALISTS` only enumerates
  the always-pre-seeded basic specialists (identification,
  event-capture). Discoverable specialists are owned by the audit
  SKILL.md in context-mill.

Generated-By: PostHog Code
Task-Id: e4fb6677-7de2-41d1-b78c-56f27096d628