fix: fully gate llama behind BRAIN_EMBED_ENABLE — postinstall, build, runtime logging #122

[four-bytes-robby]

Closes #122

Changes

• postinstall: gated behind BRAIN_EMBED_ENABLE — no binary copy without opt-in
• build.ts: @node-llama-cpp/* external only when BRAIN_EMBED_ENABLE is set
• embeddingService.ts: defensive logging at initialize() entry, env bail-out, and successful init

Verification

• bun run build succeeds
• Without BRAIN_EMBED_ENABLE, postinstall is a no-op
• initialize() logs env var state + caller for debugging