feat(skills): GitHub PAT support for private skill repos + delete by source

- Add optional authToken to skill import for GitHub private repos
- Store PAT as encrypted company secret (skill-pat:{skillId})
- Thread auth token through ghFetch, fetchText, fetchJson, and all GitHub resolution functions
- Add PATCH /companies/:companyId/skills/:skillId/auth for managing PAT per skill
- Add DELETE /companies/:companyId/skills/by-source for bulk deleting skills from a repo
- Preserve sourceAuthSecretId across skill re-imports/updates
- UI: Add PAT input field in import form for GitHub URLs
- UI: Add SkillAuthSection with ShieldCheck icon for viewing/updating/removing PAT
- UI: Add trash icon next to source label for delete-by-source

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/__tests__/company-skills-routes.test.ts

@@ -89,6 +89,7 @@ describe("company skill mutation permissions", () => {
     expect(mockCompanySkillService.importFromSource).toHaveBeenCalledWith(
       "company-1",
       "https://github.com/vercel-labs/agent-browser",
+      undefined,
     );
   });
 
@@ -266,6 +267,7 @@ describe("company skill mutation permissions", () => {
     expect(mockCompanySkillService.importFromSource).toHaveBeenCalledWith(
       "company-1",
       "https://github.com/vercel-labs/agent-browser",
+      undefined,
     );
   });