chore: add CLAUDE.md and configure nix dev shell

Add CLAUDE.md with project guidance for Claude Code including
architecture overview, build commands, and testing procedures.

Update flake.nix with proper Go development shell (go, gopls,
gotools, golangci-lint, govulncheck, delve) and buildGoModule
package definition.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1770197578,
+        "narHash": "sha256-AYqlWrX09+HvGs8zM6ebZ1pwUqjkfpnv8mewYwAo+iM=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "00c21e4c93d963c50d4c0c89bfa84ed6e0694df2",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}