Add linked issue guidance and ELI5 sections to PR generation prompts (#12613)

* Add linked issue guidance and ELI5 sections to PR generation prompts

Include linked GitHub issues in PR descriptions with Fixes/Refs guidance, and require ELI5 Problem and Solution sections before implementation details. Tests verify linked issue substitution and prompt structure enforcement.

* Include linked issue details in PR description generation

- Fetch the linked GitHub/GitLab issue title and body so generated PRs reference real issue context instead of just a number
- Use provider-specific reference syntax (Fixes/Refs, Closes/Related to, AB#) and label the issue by the active provider
- Feed issue title and description into the generation prompt while treating them as untrusted context, never as instructions
- Fall back to a cached work-item title when the provider lookup fails, and skip cross-provider issue attachment

src/main/github/issues.test.ts

@@ -111,11 +111,15 @@ describe('issue source operations', () => {
         title: 'Use upstream issues',
         state: 'open',
         html_url: 'https://github.com/stablyai/orca/issues/923',
-        labels: []
+        labels: [],
+        body: 'The issue body'
       })
     })
 
-    await expect(getIssue('/repo-root', 923)).resolves.toMatchObject({ number: 923 })
+    await expect(getIssue('/repo-root', 923)).resolves.toMatchObject({
+      number: 923,
+      description: 'The issue body'
+    })
     expect(ghExecFileAsyncMock).toHaveBeenCalledWith(
       ['api', '--cache', '300s', 'repos/stablyai/orca/issues/923'],
       { cwd: '/repo-root', host: 'github.com' }

src/main/github/issues.ts

@@ -87,7 +87,7 @@ export async function getIssue(
     }
     // Fallback for non-GitHub remotes
     const { stdout } = await ghExecFileAsync(
-      ['issue', 'view', String(issueNumber), '--json', 'number,title,state,url,labels'],
+      ['issue', 'view', String(issueNumber), '--json', 'number,title,state,url,labels,body'],
       ghOptions
     )
     const data = JSON.parse(stdout)

src/main/github/mappers.ts

@@ -133,13 +133,15 @@ export function mapIssueInfo(data: {
   url?: string
   html_url?: string
   labels?: { name: string }[]
+  body?: string | null
 }): IssueInfo {
   return {
     number: data.number,
     title: data.title,
     state: data.state?.toLowerCase() === 'open' ? 'open' : 'closed',
     url: data.html_url ?? data.url ?? '',
-    labels: (data.labels || []).map((l) => l.name)
+    labels: (data.labels || []).map((l) => l.name),
+    ...(typeof data.body === 'string' ? { description: data.body } : {})
   }
 }