Add DELETE endpoint for company skills and fix skills.sh URL resolution

- Add DELETE /api/companies/:companyId/skills/:skillId endpoint with same
  permission model as other skill mutations. Deleting a skill removes it
  from the DB, cleans up materialized runtime files, and automatically
  strips it from any agent desiredSkills that reference it.
- Fix parseSkillImportSourceInput to detect skills.sh URLs
  (e.g. https://skills.sh/org/repo/skill) and resolve them to the
  underlying GitHub repo + skill slug, instead of fetching the HTML page.
- Add tests for skills.sh URL resolution with and without skill slug.

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

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

@@ -45,6 +45,24 @@ describe("company skill import source parsing", () => {
     expect(parsed.requestedSkillSlug).toBe("find-skills");
   });
 
+  it("resolves skills.sh URL with org/repo/skill to GitHub repo", () => {
+    const parsed = parseSkillImportSourceInput(
+      "https://skills.sh/google-labs-code/stitch-skills/design-md",
+    );
+
+    expect(parsed.resolvedSource).toBe("https://github.com/google-labs-code/stitch-skills");
+    expect(parsed.requestedSkillSlug).toBe("design-md");
+  });
+
+  it("resolves skills.sh URL with org/repo (no skill) to GitHub repo", () => {
+    const parsed = parseSkillImportSourceInput(
+      "https://skills.sh/vercel-labs/skills",
+    );
+
+    expect(parsed.resolvedSource).toBe("https://github.com/vercel-labs/skills");
+    expect(parsed.requestedSkillSlug).toBeNull();
+  });
+
   it("parses skills.sh commands whose requested skill differs from the folder name", () => {
     const parsed = parseSkillImportSourceInput(
       "npx skills add https://github.com/remotion-dev/skills --skill remotion-best-practices",