feat(elaborate): add domain discovery, spec validation, and deep research to elaboration

The elaboration skill was producing shallow specs that led builders to
misinterpret intent. This overhaul adds:

- Phase 2.5: Mandatory domain discovery with active technical exploration
  (API introspection, codebase reading, web research, MCP tools)
- Phase 5.75: Spec validation gate requiring user confirmation that specs
  are detailed enough for a zero-context builder
- Richer unit templates with domain entities, data sources, technical
  specification, and explicit boundaries
- Domain model section in intent.md template
- Research tools: Task, WebSearch, WebFetch, ToolSearch, read-only MCP
  patterns (get*, list*, search*, query*, fetch*, etc.)
- Guidance to communicate findings incrementally, not dump a report
- Good/bad unit description examples showing vague vs precise specs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jwaldrip