Require cargo-zigbuild, remove cross-linker fallback

- Make zigbuild a hard requirement instead of optional with fallback
- Remove configure_cross_linker and all system linker detection logic
- Remove apt-get install of system cross-linkers from CI
- Remove unused `which` dependency
- Fail fast with clear install instructions if zigbuild is missing

https://claude.ai/code/session_01EcsZDNeWn56wFqryb4Wq7r

Cargo.toml

@@ -26,7 +26,6 @@ tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 dirs = "6.0"
 toml = "0.9"
-which = "8.0"
 base64 = "0.22"
 
 [dev-dependencies]