[codex] Add foundation/spec compiler scaffolding and eval runner

[jeevanpillay]

What Changed

This PR adds the first foundation/spec compiler and eval system for the skills repo.

• Adds foundation-creator as a separate skill lane for thesis, primitive, and company/product foundation documents.
• Adds BAML compiler contracts for both foundation-creator and spec-creator.
• Adds fixture-driven eval packets for foundation creation, spec creation, update-mode edits, ambiguous founder notes, and non-dev-domain care coordination.
• Migrates local eval execution to Bun with .env loading and Vercel AI Gateway via AI_GATEWAY_API_KEY.
• Adds a modular TypeScript eval runner with local JSON artifacts as the source of truth.
• Adds deterministic validators for template shape, update preservation, open questions, taxonomy metadata, and source-bound document constraints.
• Adds Braintrust-ready reporting while keeping local artifacts canonical.
• Adds CI smoke coverage for static checks plus live foundation/spec smoke evals.
• Upgrades CI/runtime targeting to Node 24 via .node-version.

Why

The goal is to make skill quality measurable before these skills are used broadly:

• Separate strategic foundation-doc generation from concrete behavioral SPEC.md generation.
• Turn messy notes into typed intermediate briefs before rendering final documents.
• Catch prompt drift with deterministic checks instead of relying only on LLM judges.
• Preserve ambiguity explicitly instead of inventing product or implementation decisions.
• Establish a reusable eval runner shape that can later support more skills and projects.

Validation

Latest local validation:

• bun run ci:check
• bun run eval:spec -- create-from-lightfast-founder-notes-packet
• bun run eval:spec:smoke passed 4/4

Latest GitHub Actions validation:

• Evals / Typecheck and smoke evals passed on run 24827490425 in 6m21s

Notes:

• The PR is intentionally not squashed; commit history reflects the iterative prompt/eval tuning process.
• The remaining Node 20 GitHub Actions annotation is informational: repository CI forces Node 24 for the affected actions.