feat(setup): install sections/ for cherry-pick targets (claude + kiro) (T9)

Two install targets cherry-pick SKILL.md and would leave a carved skill's
sections/ behind, 404ing a runtime 'Read sections/<name>.md':
- link_claude_skill_dirs: link the sections/ subdir via _link_or_copy (windows
  gets a fresh copy on every ./setup)
- kiro per-skill loop: sed-rewrite + copy each sections/* so paths resolve under
  ~/.kiro, not ~/.codex/~/.claude

codex/factory/opencode link the whole generated dir, so sections ride free.
Addresses Codex outside-voice #4/#6 (runtime pathing landmine). Inert until a
skill is carved. Static-tripwire test + windows-fallback invariant green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

setup

@@ -474,6 +474,14 @@ link_claude_skill_dirs() {
       # Validate target isn't a symlink before creating the link
       if [ -L "$target/SKILL.md" ]; then rm "$target/SKILL.md"; fi
       _link_or_copy "$gstack_dir/$dir_name/SKILL.md" "$target/SKILL.md"
+      # Link the sections/ subdir for carved skills (v2 plan T9). The prefixed
+      # Claude skill dir otherwise holds only SKILL.md, so a runtime
+      # "Read sections/<name>.md" 404s. Route through _link_or_copy so Windows
+      # gets a fresh copy (and re-copies on every ./setup, refreshing staleness).
+      if [ -d "$gstack_dir/$dir_name/sections" ]; then
+        if [ -e "$target/sections" ] || [ -L "$target/sections" ]; then rm -rf "$target/sections"; fi
+        _link_or_copy "$gstack_dir/$dir_name/sections" "$target/sections"
+      fi
       linked+=("$link_name")
     fi
   done
@@ -1049,6 +1057,20 @@ if [ "$INSTALL_KIRO" -eq 1 ]; then
           -e "s|~/.codex/skills/gstack|~/.kiro/skills/gstack|g" \
           -e "s|~/.claude/skills/gstack|~/.kiro/skills/gstack|g" \
           "$skill_dir/SKILL.md" > "$target_dir/SKILL.md"
+      # Carved skills (v2 plan T9): rewrite + copy each sections/*.md the same way,
+      # so a runtime "Read sections/<name>.md" resolves under ~/.kiro and doesn't
+      # leak a ~/.codex or ~/.claude path. Kiro builds from the codex output, so
+      # these section files only exist for skills that have been carved.
+      if [ -d "$skill_dir/sections" ]; then
+        mkdir -p "$target_dir/sections"
+        for section_file in "$skill_dir/sections"/*; do
+          [ -f "$section_file" ] || continue
+          sed -e 's|\$HOME/.codex/skills/gstack|$HOME/.kiro/skills/gstack|g' \
+              -e "s|~/.codex/skills/gstack|~/.kiro/skills/gstack|g" \
+              -e "s|~/.claude/skills/gstack|~/.kiro/skills/gstack|g" \
+              "$section_file" > "$target_dir/sections/$(basename "$section_file")"
+        done
+      fi
     done
     echo "gstack ready (kiro)."
     echo "  browse: $BROWSE_BIN"