feat: add per-project dev-shell skills placement (task 0003)

Expose lib.mkSkillsShellHook, which takes a list of selected skill derivations
and returns a dev-shell shellHook string. On shell entry the hook symlinks each
selected skill as a direct child of the project's .claude/skills/, pointing into
the store. It is stateless: each entry first removes only store-pointing symlinks
(deselected skills included), then relinks the current selection, leaving
hand-authored real directories untouched and regenerating a self-ignoring
.gitignore of the managed names. Add a nix flake check that sources the produced
hook against a fixture project and asserts placement, stale-removal, the
collision guard, non-store-symlink survival, and the .gitignore contents.

checks/shell-hook.nix

@@ -0,0 +1,121 @@
+# Runs the dev-shell helper's hook against a fixture project and asserts the
+# placement, stateless stale-removal, and hand-authored-skill safety it promises.
+{
+  pkgs,
+  mkSkill,
+  mkSkillsShellHook,
+}:
+let
+  # Sample skills built through the flake's own `mkSkill`, so they carry the
+  # `skillName` passthru the hook reads and land at real store paths the hook
+  # can link to.
+  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";
+  # Selected, but a hand-authored real directory of the same name already exists
+  # in the fixture, so the hook must skip it.
+  skillGamma = mkSampleSkill "sample-gamma";
+
+  # Two hooks for two successive selections: the second drops sample-beta, so
+  # sourcing it after the first proves a deselected skill's symlink is removed.
+  hookBoth = mkSkillsShellHook [
+    skillAlpha
+    skillBeta
+    skillGamma
+  ];
+  hookAlpha = mkSkillsShellHook [ skillAlpha ];
+in
+pkgs.runCommandLocal "skills-shell-hook-check"
+  {
+    inherit hookBoth hookAlpha;
+    alphaPath = "${skillAlpha}";
+    betaPath = "${skillBeta}";
+  }
+  ''
+    fail() { echo "FAIL: $1" >&2; exit 1; }
+
+    echo "the hook is a non-empty string referencing each skill's store path and name"
+    [ -n "$hookBoth" ] || fail "the produced hook is empty"
+    for token in "$alphaPath" "$betaPath" sample-alpha sample-beta; do
+      printf '%s' "$hookBoth" | grep -qF "$token" \
+        || fail "the hook does not reference $token"
+    done
+
+    # The fixture project carries three things the hook must not disturb: a
+    # hand-authored skill under its own name, a hand-authored skill whose name
+    # collides with a selected one, and a symlink pointing outside the store.
+    mkdir -p proj/.claude/skills/handmade
+    printf '%s\n' "a hand-authored skill" > proj/.claude/skills/handmade/SKILL.md
+    mkdir -p proj/.claude/skills/sample-gamma
+    printf '%s\n' "a hand-authored gamma" > proj/.claude/skills/sample-gamma/SKILL.md
+    ln -s /nonexistent/external-target proj/.claude/skills/external
+    cd proj
+
+    echo "first entry: the two free skills link as store symlinks that resolve"
+    printf '%s\n' "$hookBoth" > ../hook-both.sh
+    . ../hook-both.sh
+    for name in sample-alpha sample-beta; do
+      [ -L ".claude/skills/$name" ] || fail "$name was not linked as a symlink"
+      case "$(readlink ".claude/skills/$name")" in
+        ${builtins.storeDir}/*) : ;;
+        *) fail "$name's symlink does not point into the store" ;;
+      esac
+      test -f ".claude/skills/$name/SKILL.md" \
+        || fail "$name's link does not resolve to its SKILL.md"
+    done
+
+    echo "a selected skill colliding with a hand-authored one leaves the real directory"
+    { [ -d .claude/skills/sample-gamma ] && [ ! -L .claude/skills/sample-gamma ]; } \
+      || fail "the colliding hand-authored skill was replaced by a symlink"
+    grep -qx "a hand-authored gamma" .claude/skills/sample-gamma/SKILL.md \
+      || fail "the colliding hand-authored skill's content was overwritten"
+
+    echo "the non-colliding hand-authored skill is an untouched real directory"
+    { [ -d .claude/skills/handmade ] && [ ! -L .claude/skills/handmade ]; } \
+      || fail "the hand-authored skill was replaced or removed"
+    test -f .claude/skills/handmade/SKILL.md || fail "the hand-authored skill lost its content"
+
+    echo "a symlink pointing outside the store is not ours, so it is left alone"
+    [ -L .claude/skills/external ] || fail "the non-store symlink was removed"
+
+    echo "the .gitignore lists only the skills the hook actually linked, plus itself"
+    grep -qx '/.gitignore' .claude/skills/.gitignore || fail ".gitignore does not ignore itself"
+    grep -qx '/sample-alpha' .claude/skills/.gitignore || fail ".gitignore omits sample-alpha"
+    grep -qx '/sample-beta' .claude/skills/.gitignore || fail ".gitignore omits sample-beta"
+    for tracked in /handmade /sample-gamma /external; do
+      grep -qx "$tracked" .claude/skills/.gitignore \
+        && fail ".gitignore lists $tracked, which would untrack a real path"
+    done
+
+    echo "no manifest or state file is written: only the fixtures and links exist"
+    entries=$(ls -A .claude/skills | sort | tr '\n' ' ')
+    [ "$entries" = ".gitignore external handmade sample-alpha sample-beta sample-gamma " ] \
+      || fail "unexpected entries under .claude/skills: [$entries]"
+
+    echo "second entry with sample-beta deselected: its symlink is removed, the rest stay"
+    printf '%s\n' "$hookAlpha" > ../hook-alpha.sh
+    . ../hook-alpha.sh
+    test ! -e .claude/skills/sample-beta || fail "the deselected skill's symlink was not removed"
+    [ -L .claude/skills/sample-alpha ] || fail "the still-selected skill was dropped"
+    { [ -d .claude/skills/handmade ] && [ ! -L .claude/skills/handmade ]; } \
+      || fail "the hand-authored skill was disturbed on re-entry"
+    { [ -d .claude/skills/sample-gamma ] && [ ! -L .claude/skills/sample-gamma ]; } \
+      || fail "the colliding hand-authored skill was disturbed on re-entry"
+    [ -L .claude/skills/external ] || fail "the non-store symlink was disturbed on re-entry"
+
+    echo "the .gitignore tracks the new selection"
+    grep -qx '/sample-alpha' .claude/skills/.gitignore || fail ".gitignore lost sample-alpha"
+    grep -qx '/sample-beta' .claude/skills/.gitignore \
+      && fail ".gitignore still lists the deselected sample-beta"
+
+    touch "$out"
+  ''