feat(github): create stacked pull requests (#13750)

Adds GitHub stacked pull request creation: a contextual "Stack this PR above #N" option that appears only when the selected base branch has an open PR, plus the main-process stack preflight and registration.

Also reworks the create-review composer for cohesion: shadcn Checkbox and Label primitives, base label above a full-width searchable combobox with attached results, keyboard navigation, and a unified field skin, spacing and typography scale.

Verified end to end against real GitHub: extending an existing stack and creating a new one.

src/main/github/github-stack-api-responses.ts

@@ -0,0 +1,52 @@
+import type { HostedReviewSummary } from '../../shared/hosted-review'
+
+export type NumberedHostedReviewSummary = Omit<HostedReviewSummary, 'number'> & { number: number }
+
+export type GitHubStackPullRequest = NumberedHostedReviewSummary & {
+  headRefName: string
+  baseRefName: string
+}
+
+export type GitHubStack = {
+  number: number
+  open: boolean
+  pull_requests: { number: number }[]
+}
+
+export function parseGitHubStackPullRequests(stdout: string): GitHubStackPullRequest[] {
+  const pullRequests = JSON.parse(stdout) as {
+    number?: unknown
+    html_url?: unknown
+    head?: { ref?: unknown }
+    base?: { ref?: unknown }
+  }[]
+  return pullRequests.flatMap((pullRequest) => {
+    const number = Number(pullRequest.number)
+    const url = typeof pullRequest.html_url === 'string' ? pullRequest.html_url : ''
+    const headRefName = typeof pullRequest.head?.ref === 'string' ? pullRequest.head.ref : ''
+    const baseRefName = typeof pullRequest.base?.ref === 'string' ? pullRequest.base.ref : ''
+    return Number.isInteger(number) && number > 0 && url && headRefName && baseRefName
+      ? [{ number, url, headRefName, baseRefName }]
+      : []
+  })
+}
+
+export function parseGitHubStacks(stdout: string): GitHubStack[] {
+  const stacks = JSON.parse(stdout) as {
+    number?: unknown
+    open?: unknown
+    pull_requests?: { number?: unknown }[]
+  }[]
+  return stacks.flatMap((stack) => {
+    const number = Number(stack.number)
+    const pullRequests = (stack.pull_requests ?? []).flatMap((pullRequest) => {
+      const pullRequestNumber = Number(pullRequest.number)
+      return Number.isInteger(pullRequestNumber) && pullRequestNumber > 0
+        ? [{ number: pullRequestNumber }]
+        : []
+    })
+    return Number.isInteger(number) && number > 0
+      ? [{ number, open: stack.open === true, pull_requests: pullRequests }]
+      : []
+  })
+}