feat: brain-only install mode — ladder without the harness

[BaseInfinity]

Problem

Users who want the model escalation ladder (Opus 4.6 max → GPT-5.5 xhigh → Fable high) but NOT the full SDLC harness (no hooks, skills, TDD enforcement, merge gates) have no install path. Today it's a hand-written CLAUDE.md section per project.

Proof of concept

~/pyro/CLAUDE.md — manually created, three-section doc:

1. Builder: Opus 4.6[1m] max (global settings)
2. First brain: GPT-5.5 xhigh via codex exec (workflow instruction)
3. Second brain: Fable high via advisor() (global settings, escalation only at <95%)

No config files beyond CLAUDE.md. Global ~/.claude/settings.json already carries model + advisorModel.

Proposal

Add a --brain-only flag (or similar) to sdlc-wizard CLI that writes ONLY the brain ladder CLAUDE.md section into a target project. No hooks, no skills, no .claude/ config beyond what global settings provide.

Skills first → wizard later: let the hand-written pyro setup prove itself in real use before templating it. This issue tracks the eventual CLI integration, not an immediate build.

Acceptance criteria

• npx sdlc-wizard --brain-only writes a brain ladder section to the target CLAUDE.md
• Does NOT install hooks, skills, or any .claude/ directory contents
• Respects existing CLAUDE.md content (append, don't overwrite)
• Documents the three-seat ladder and when to escalate