Read skills from the global and workspace .agents/skills directories

- Discover skill directories under ~/.agents/skills and each
  workspace''s .agents/skills; a directory qualifies with a full
  skill.manifest.json or just a SKILL.md, whose manifest is derived
  (frontmatter name or the directory name). Paths resolve per platform
  on Windows, macOS, and Linux.
- Sync at startup for every organization and again when the Skills
  page opens; skills the org already carries are skipped, and one
  broken directory cannot block the rest.
- AGENTCOMPANY_NEW_SKILLS_ROOT points tests at a temporary library
  (overriding USERPROFILE crashes Electron on Windows).

tests/e2e/screenshots.spec.ts

@@ -9,7 +9,12 @@ test("captures every page for visual review", async () => {
   mkdirSync(profileDir, { recursive: true });
   const app = await electron.launch({
     args: [resolve("out/main/index.js")],
-    env: { ...process.env, AGENTCOMPANY_NEW_PROFILE_DIR: profileDir, AGENTCOMPANY_NEW_AUTORUN: "0" },
+    env: {
+      ...process.env,
+      AGENTCOMPANY_NEW_PROFILE_DIR: profileDir,
+      AGENTCOMPANY_NEW_AUTORUN: "0",
+      AGENTCOMPANY_NEW_SKILLS_ROOT: join(profileDir, "skills-root"),
+    },
   });
   const outDir = resolve("test-results/screenshots");
   mkdirSync(outDir, { recursive: true });