Fix all shellcheck warnings for nix build gate

- SC2015: Wrap A && B || C patterns in brace groups for directive scope
- SC2064: Suppress for intentional early trap expansion (local vars)
- SC2164: Add || exit to cd commands in subshells
- SC2001: Suppress for sed URL injection (clearer than parameter expansion)
- SC1083: Handle {tree} git syntax (quote or suppress)
- SC1091: Suppress for runtime-resolved source paths
- SC2034: Remove unused exit codes (E_OK, E_CONFIG, E_AUTH)
- SC2116: Eliminate useless echo in mono_auth_url construction

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1770562336,
+        "narHash": "sha256-ub1gpAONMFsT/GU2hV6ZWJjur8rJ6kKxdm9IlCT0j84=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "d6c71932130818840fc8fe9509cf50be8c64634f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}