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.

checks/home-manager-module.nix

@@ -0,0 +1,154 @@
+# Builds the home-files tree `homeModules.default` produces under several
+# operator configurations and asserts on the skills each one places.
+{
+  pkgs,
+  home-manager,
+  module,
+  mkSkill,
+}:
+let
+  inherit (pkgs) lib;
+
+  # A sample skill built through the flake's own `mkSkill`, so it carries the
+  # `skillName` passthru the module reads to place it.
+  mkSampleSkill =
+    name:
+    mkSkill {
+      inherit pkgs name;
+      src = pkgs.runCommandLocal "${name}-src" { } ''
+        mkdir -p "$out"
+        printf '%s\n' "sample skill ${name}" > "$out/SKILL.md"
+      '';
+    };
+
+  skillAlpha = mkSampleSkill "sample-alpha";
+  skillBeta = mkSampleSkill "sample-beta";
+
+  # A skill the operator declares as their own attribute-set entry, so the
+  # attribute-set assertion can look for it beside the module's.
+  operatorAttrSkill = pkgs.runCommandLocal "operator-attr-skill" { } ''
+    mkdir -p "$out"
+    printf '%s\n' "the operator's own attribute-set skill" > "$out/SKILL.md"
+  '';
+
+  # A whole directory of skills, one folder per skill, for the path form of the
+  # operator's own `programs.claude-code.skills`.
+  operatorSkillsDir = pkgs.runCommandLocal "operator-skills-dir" { } ''
+    mkdir -p "$out/operator-dir-skill"
+    printf '%s\n' "the operator's own whole-directory skill" \
+      > "$out/operator-dir-skill/SKILL.md"
+  '';
+
+  # Evaluate the real module through home-manager's standalone entry point and
+  # return the home-files tree it would link into $HOME.
+  homeFiles =
+    operatorConfig:
+    (home-manager.lib.homeManagerConfiguration {
+      inherit pkgs;
+      modules = [
+        module
+        {
+          home.username = "operator";
+          home.homeDirectory = "/home/operator";
+          home.stateVersion = "24.11";
+        }
+        operatorConfig
+      ];
+    }).config.home-files;
+
+  # Two skills selected beside the operator's own attribute-set skill, with
+  # Claude Code enabled.
+  attrSetSkills = homeFiles {
+    programs.claude-code.enable = true;
+    programs.agents.skills = [
+      skillAlpha
+      skillBeta
+    ];
+    # A store path, not a bare derivation, so the value takes the option's path
+    # branch rather than its attribute-set branch.
+    programs.claude-code.skills.operator-attr-skill = "${operatorAttrSkill}";
+  };
+
+  # The operator's own skills as a whole directory (path form).
+  # Setting this single-valued option to a path also proves the module never
+  # contributes to it, since an attribute-set contribution would clash with the
+  # path at eval.
+  wholeDirSkills = homeFiles {
+    programs.claude-code.enable = true;
+    programs.agents.skills = [ skillAlpha ];
+    programs.claude-code.skills = "${operatorSkillsDir}";
+  };
+
+  # A non-default `configDir`, so the assertions can prove placement follows it
+  # rather than a hardcoded `.claude`.
+  customConfigDir = homeFiles {
+    programs.claude-code.enable = true;
+    programs.claude-code.configDir = "/home/operator/.config/claude";
+    programs.agents.skills = [ skillAlpha ];
+  };
+
+  # Claude Code disabled, with a skill still selected.
+  claudeCodeOff = homeFiles {
+    programs.claude-code.enable = false;
+    programs.agents.skills = [ skillAlpha ];
+  };
+
+  # An empty selection with Claude Code enabled.
+  emptySelection = homeFiles {
+    programs.claude-code.enable = true;
+    programs.agents.skills = [ ];
+  };
+in
+pkgs.runCommandLocal "skills-home-manager-module-check"
+  {
+    # Forcing each derivation as a build input builds its home-files tree, so an
+    # eval-level composition failure surfaces before any assertion runs.
+    inherit
+      attrSetSkills
+      wholeDirSkills
+      customConfigDir
+      claudeCodeOff
+      emptySelection
+      ;
+  }
+  ''
+    fail() { echo "FAIL: $1" >&2; exit 1; }
+
+    echo "attribute-set skills: both selected skills and the operator's own land"
+    test -f "$attrSetSkills/.claude/skills/sample-alpha/SKILL.md" \
+      || fail "selected skill sample-alpha did not land at its own name"
+    test -f "$attrSetSkills/.claude/skills/sample-beta/SKILL.md" \
+      || fail "selected skill sample-beta did not land at its own name"
+    test -f "$attrSetSkills/.claude/skills/operator-attr-skill/SKILL.md" \
+      || fail "the operator's own attribute-set skill was clobbered"
+
+    echo "recursive placement: a skill is a real directory of per-file symlinks"
+    # A file-existence test follows symlinks, so it cannot tell a recursive tree
+    # from one opaque symlink over the whole skill.
+    # The entry's own type is what the assertion reads instead.
+    test -d "$attrSetSkills/.claude/skills/sample-alpha" \
+      && test ! -L "$attrSetSkills/.claude/skills/sample-alpha" \
+      || fail "sample-alpha was placed as an opaque symlink, not a recursive tree"
+
+    echo "whole-directory skills: the selected skill lands beside the operator's directory"
+    test -f "$wholeDirSkills/.claude/skills/sample-alpha/SKILL.md" \
+      || fail "selected skill did not coexist with a whole skills directory"
+    test -f "$wholeDirSkills/.claude/skills/operator-dir-skill/SKILL.md" \
+      || fail "the operator's whole skills directory was clobbered"
+
+    echo "custom configDir: placement follows configDir rather than a hardcoded .claude"
+    test -f "$customConfigDir/.config/claude/skills/sample-alpha/SKILL.md" \
+      || fail "skill was not placed under the configured configDir"
+    test ! -e "$customConfigDir/.claude/skills/sample-alpha" \
+      || fail "skill was placed under a hardcoded .claude, ignoring configDir"
+
+    echo "Claude Code disabled: no skill entry is written despite a selection"
+    test ! -e "$claudeCodeOff/.claude/skills/sample-alpha" \
+      || fail "a skill was placed with claude-code disabled"
+
+    echo "empty selection: nothing is installed"
+    test ! -e "$emptySelection/.claude/skills/sample-alpha" \
+      || fail "an unselected skill was placed"
+
+    touch "$out"
+  ''