feat(bmad-spec): add Spec kernel distiller skill
New 2-plan-workflows skill that distills any intent input (brain dump,
PRD, transcript, brief) into a spec.md carrying the five-field kernel:
Problem, Capabilities, Constraints, Non-goals, Success signal. Headless
callers receive JSON; interactive runs close conversationally with the
spec path and gap-coverage invitations.
Includes:
- SKILL.md with activation contract and conventions
- customize.toml exposing template path, output path, run-folder pattern
- assets/spec-template.md (five-field skeleton)
- assets/headless-schemas.md (JSON IO contracts)
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