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/shared/ipc.ts

@@ -33,6 +33,7 @@ export const IPC_CHANNELS = {
   streamProviderResponse: "agentcompany:stream-provider-response",
   cancelProviderResponse: "agentcompany:cancel-provider-response",
   importSkillDirectory: "agentcompany:import-skill-directory",
+  syncSkillsFromDisk: "agentcompany:sync-skills-from-disk",
   bindSkill: "agentcompany:bind-skill",
   resolveAgentSkills: "agentcompany:resolve-agent-skills",
   runSkillTests: "agentcompany:run-skill-tests",