feat(skills): GitHub PAT support for private skill repos

- Add optional authToken to skill import for GitHub private repos
- Store PAT as encrypted company secret (skill-pat:{skillId})
- Thread auth token through ghFetch and GitHub resolution helpers
- Add PATCH /companies/:companyId/skills/:skillId/auth for managing PAT per skill
- Preserve sourceAuthSecretId across skill re-imports/updates
- Delete PAT secret on PAT clear and on skill deletion to prevent orphans
- UI: Add PAT input field in import form for GitHub URLs
- UI: Add SkillAuthSection with ShieldCheck icon for viewing/updating/removing PAT

server/src/services/github-fetch.ts

@@ -16,9 +16,13 @@ export function resolveRawGitHubUrl(hostname: string, owner: string, repo: strin
     : `https://${hostname}/raw/${owner}/${repo}/${ref}/${p}`;
 }
 
-export async function ghFetch(url: string, init?: RequestInit): Promise<Response> {
+export async function ghFetch(url: string, init?: RequestInit, authToken?: string): Promise<Response> {
+  const headers = new Headers(init?.headers);
+  if (authToken) {
+    headers.set("Authorization", `Bearer ${authToken}`);
+  }
   try {
-    return await fetch(url, init);
+    return await fetch(url, { ...init, headers });
   } catch {
     throw unprocessable(`Could not connect to ${new URL(url).hostname} — ensure the URL points to a GitHub or GitHub Enterprise instance`);
   }