bild: refactor symlink structure (_/nix/Ns.hs → store path)

Refactor bild symlink structure to be cleaner:

Current

• _/nix/Omni/Task.hs/task → /nix/store/xxx-task
• Access binary via _/nix/Omni/Task.hs/task/bin/task (awkward)

Proposed

• _/nix/Omni/Task.hs → /nix/store/xxx-task
• Access binary via _/nix/Omni/Task.hs/bin/task (natural)

This keeps the store closure nested under the repo namespace cleanly.

Changes needed

1. Modify nixBuild symlink logic to point namespace dir at store path 2. Update all code that reads from _/nix/Namespace/outname 3. Update local rundep resolution accordingly