feat: add global-placement home-manager module (task 0002)

Expose homeModules.default, a home-manager module whose programs.agents.skills
option places operator-selected skill derivations into Claude Code's skills
directory, one recursive home.file per skill, gated on programs.claude-code.enable
and sourcing configDir from the claude-code module. Add a nix flake check that
builds the home-files tree under several operator configurations and asserts
placement, recursion, the enable gate, configDir sourcing, and coexistence with
an operator's own skills.

flake.lock

@@ -18,6 +18,26 @@
         "type": "github"
       }
     },
+    "home-manager": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1784725727,
+        "narHash": "sha256-J5+C9wsO0lhDyUalQzfplDbRjyHDYeEH5+9sdyXtwa8=",
+        "owner": "nix-community",
+        "repo": "home-manager",
+        "rev": "041a999e8c1c5b731913855909e68d30ca69b8e0",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "home-manager",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1784497964,
@@ -37,6 +57,7 @@
     "root": {
       "inputs": {
         "flake-utils": "flake-utils",
+        "home-manager": "home-manager",
         "nixpkgs": "nixpkgs"
       }
     },