feat(#824): protect installer skill links

packages/mosaic/framework/tools/_scripts/mosaic-sync-skills

@@ -161,6 +161,7 @@ link_targets=(
 )
 
 canonical_real="$(readlink -f "$MOSAIC_SKILLS_DIR")"
+local_real="$(readlink -f "$MOSAIC_LOCAL_SKILLS_DIR")"
 
 # Build an associative array from the colon-separated whitelist for O(1) lookup.
 # When MOSAIC_INSTALL_SKILLS is empty, all skills are allowed.
@@ -203,7 +204,14 @@ link_skill_into_target() {
   link_path="$target_dir/$name"
 
   if [[ -L "$link_path" ]]; then
-    ln -sfn "$skill_path" "$link_path"
+    local raw_target resolved_target
+    raw_target="$(readlink "$link_path")"
+    resolved_target="$(node -e 'const p=require("node:path"); process.stdout.write(p.resolve(p.dirname(process.argv[1]), process.argv[2]));' "$link_path" "$raw_target")"
+    if [[ "$resolved_target" == "$canonical_real/"* || "$resolved_target" == "$local_real/"* ]]; then
+      ln -sfn "$skill_path" "$link_path"
+    else
+      echo "[mosaic-skills] Preserve foreign runtime symlink: $link_path"
+    fi
     return
   fi
 

packages/mosaic/framework/tools/_scripts/mosaic-sync-skills.ps1

@@ -79,9 +79,26 @@ function Link-SkillIntoTarget {
 
     $linkPath = Join-Path $TargetDir $name
 
-    # Already a junction/symlink — recreate
+    # Recreate only Mosaic-owned junctions/symlinks. Foreign reparse points are
+    # runtime-owned and must never be clobbered by install/upgrade auto-sync.
     $existing = Get-Item $linkPath -Force -ErrorAction SilentlyContinue
     if ($existing -and ($existing.Attributes -band [System.IO.FileAttributes]::ReparsePoint)) {
+        $rawTarget = @($existing.Target)[0]
+        $candidate = if ([System.IO.Path]::IsPathRooted($rawTarget)) {
+            $rawTarget
+        }
+        else {
+            Join-Path (Split-Path $linkPath -Parent) $rawTarget
+        }
+        $resolvedTarget = [System.IO.Path]::GetFullPath($candidate)
+        $canonicalRoot = [System.IO.Path]::GetFullPath($MosaicSkillsDir).TrimEnd('\') + '\'
+        $localRoot = [System.IO.Path]::GetFullPath($MosaicLocalSkillsDir).TrimEnd('\') + '\'
+        $owned = $resolvedTarget.StartsWith($canonicalRoot, [System.StringComparison]::OrdinalIgnoreCase) -or
+            $resolvedTarget.StartsWith($localRoot, [System.StringComparison]::OrdinalIgnoreCase)
+        if (-not $owned) {
+            Write-Host "[mosaic-skills] Preserve foreign runtime symlink: $linkPath"
+            return
+        }
         Remove-Item $linkPath -Force
     }
     elseif ($existing) {

packages/mosaic/src/commands/skill.spec.ts

@@ -1,5 +1,7 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { Command } from 'commander';
+import { spawnSync } from 'node:child_process';
+import { fileURLToPath } from 'node:url';
 import {
   existsSync,
   lstatSync,
@@ -21,6 +23,10 @@ import {
   type SkillPaths,
 } from './skill.js';
 
+const LEGACY_SYNC_SCRIPT = fileURLToPath(
+  new URL('../../framework/tools/_scripts/mosaic-sync-skills', import.meta.url),
+);
+
 describe('Claude skill bridge', () => {
   let root: string;
   let paths: SkillPaths;
@@ -235,6 +241,33 @@ describe('Claude skill bridge', () => {
     });
   });
 
+  describe('install linker compatibility', () => {
+    it('preserves live and dangling foreign Claude symlinks while linking missing skills', () => {
+      createSkill('dangling-foreign');
+      createSkill('live-foreign');
+      createSkill('missing');
+      mkdirSync(paths.claudeSkillsDir, { recursive: true });
+      const external = join(root, 'external');
+      mkdirSync(external);
+      const liveLink = join(paths.claudeSkillsDir, 'live-foreign');
+      const danglingLink = join(paths.claudeSkillsDir, 'dangling-foreign');
+      symlinkSync(external, liveLink);
+      symlinkSync(join(root, 'external-missing'), danglingLink);
+
+      const result = spawnSync('bash', [LEGACY_SYNC_SCRIPT, '--link-only'], {
+        encoding: 'utf8',
+        env: { ...process.env, HOME: root, MOSAIC_HOME: join(root, '.config', 'mosaic') },
+      });
+
+      expect(result.status, result.stderr).toBe(0);
+      expect(readlinkSync(liveLink)).toBe(external);
+      expect(readlinkSync(danglingLink)).toBe(join(root, 'external-missing'));
+      expect(readlinkSync(join(paths.claudeSkillsDir, 'missing'))).toBe(
+        join(paths.mosaicSkillsDir, 'missing'),
+      );
+    });
+  });
+
   describe('syncClaudeSkills', () => {
     it('generically creates every missing canonical link and repairs managed broken links', () => {
       createSkill('added-after-setup');