feat: scan workspace .agents/skills/ directories for skill imports

Add scanWorkspaceSkills method to skill service that iterates all
workspace local paths for a company and imports SKILL.md files from
.agents/skills/ subdirectories. Uses path.join for cross-platform
compatibility (Windows/macOS/Linux). The scan runs automatically
when skills are listed via IPC, with graceful error handling per
workspace so one bad path does not block the entire listing.

Also adds listWorkspaceLocalPaths to database for retrieving workspace
paths by company.

src/main/database.ts

@@ -1182,6 +1182,13 @@ export class AppDatabase {
     return row ? { ...row, isPrimary: toBoolean(row.isPrimary) } : null;
   }
 
+  listWorkspaceLocalPaths(companyId: string): string[] {
+    const rows = this.db
+      .prepare("select local_path as localPath from workspaces where company_id = ? and local_path is not null")
+      .all(companyId) as Array<{ localPath: string }>;
+    return rows.map((r) => r.localPath).filter(Boolean);
+  }
+
   getAnyCompanyWorkspace(companyId: string): WorkspaceRecord | null {
     const row = this.db
       .prepare(