refactor(completion): generate shell completions from comptime metadata

Replace hardcoded zsh/bash completion scripts with dynamic generation
from a single CmdMeta table. The exhaustive switch on cli.Command
ensures new commands trigger a compile error until their completion
metadata is added.

Also add link_libc=true to build.zig for mirror_probe's POSIX socket
calls, fixing cross-compilation errors on CI (x86_64-linux).

Author: lispking

build.zig

@@ -37,6 +37,7 @@ pub fn build(b: *std.Build) void {
             .root_source_file = b.path("src/main.zig"),
             .target = target,
             .optimize = optimize,
+            .link_libc = true,
         }),
     });