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).

src/preload/index.ts

@@ -56,6 +56,7 @@ const api: DesktopApi = {
   streamProviderResponse: (input) => invoke(IPC_CHANNELS.streamProviderResponse, input),
   cancelProviderResponse: (input) => invoke(IPC_CHANNELS.cancelProviderResponse, input),
   importSkillDirectory: (input) => invoke(IPC_CHANNELS.importSkillDirectory, input),
+  syncSkillsFromDisk: (input) => invoke(IPC_CHANNELS.syncSkillsFromDisk, input),
   bindSkill: (input) => invoke(IPC_CHANNELS.bindSkill, input),
   resolveAgentSkills: (input) => invoke(IPC_CHANNELS.resolveAgentSkills, input),
   runSkillTests: (input) => invoke(IPC_CHANNELS.runSkillTests, input),