Integrate weeder for Haskell dead code detection

Integrate dead code detection for Haskell into the build/lint toolchain.

Problem: Haskell codebases accumulate dead code over time - unused functions, orphaned modules, etc. No automated way to detect this currently.

Solution: Integrate [weeder](https://github.com/ocharles/weeder) or similar tool.

Options for integration: 1. Omni/Lint.hs - run as part of lint command 2. Omni/Bild.hs - run as post-build check (weeder needs HIE files from build)

Notes:

• Weeder requires HIE files, so it runs after compilation
• May need to configure which roots are considered "alive" (main, test, exported API)
• Could be opt-in flag initially (bild --weeder or lint --dead-code)

References:

• https://github.com/ocharles/weeder
• https://hackage.haskell.org/package/weeder