SKILL.md cliff-edge map

CI cap is 500 lines per tests/skills/test-skill-length.sh. This page shows where every >300-line SKILL.md sits relative to the cliff, and what this PR moved.

Cap: 500 Warn: 400 Skills: 107 This PR: brainstorm 500 → 419 (81 lines of headroom recovered)
Cliff (≥495, <6 lines from cap) Danger (≥480) Warn (>400) OK (≤400)
SkillLinesHeadroom to capStatus
What this PR changed

Three sections moved out of src/skills/brainstorm/SKILL.md into references/ files:

Net: 500 → 419 lines. The skill still functions identically — references load on demand via Read("${CLAUDE_SKILL_DIR}/references/<file>").

Three other skills are still on the cliff: cover (495), review-pr (490), implement (487). They follow the same bloat pattern (CC 2.1.108+ procedural compliance text) and should get the same treatment in follow-up PRs.

Why this matters

When a skill sits at the line cap, every commit that touches it can re-break CI. Before this PR, brainstorm had 0 lines of headroom — it broke from a single trailing newline. The previous fix (#1576) trimmed exactly 1 line and merged because the queue was blocked, but it didn't move the cliff.

This PR moves the cliff: brainstorm now has 81 lines of headroom and stops being a single-keystroke risk.