[P4] Remove dead code

[Bccorb]

Severity: P4 — Cleanup

Where:

• src/utils/writeEnv.ts (no callers)
• src/generators/docker/docker.ts buildJWKSConfig/generateKid + src/core/secrets.ts (test-only)
• src/generators/auth/auth.ts setupDockerAuth (unreachable — generateAuthServer is only called with "local") and its context: any, mode: …|Symbol signature
• src/generators/admin/admin.ts (empty) + its placeholder test
• redundant fs.mkdirSync(apiDir) in wireApiEnv (init.ts:338)

Suggested fix
Delete unused modules/branches and tighten generateAuthServer's signature.

Filed from an internal code audit (2026-07). File references are against main at audit time.