Create SPEC.md

Generates SPEC.md — the Step 3 checkpoint of the GSDL pipeline: Pick up source → Brainstorm → SPEC.md → Slices → Review → Branch → Execute → Verify → PR + bot → Close loop

SPEC.md is the single, canonical plan document for the project. It is designed to be clear and actionable enough for a junior developer — or an execution subagent — to implement from, and detailed enough for a human reviewer to approve or reject at the Step 5 checkpoint.

Tier

LARGE. This step absorbs Step 2 (brainstorm / clarify intent): asking the questions that actually matter, surfacing tradeoffs the source didn't consider, and turning fuzzy intent into a plan every downstream MEDIUM subagent implements against. Errors here propagate through the whole pipeline.