feat(cli): add pull request creation with AI-generated messages (#2)

Add a new `gai pr` subcommand that generates pull request titles and descriptions using AI, then creates the PR via GitHub CLI (`gh`) or Gitea CLI (`tea`). This extends the existing commit-generation system by reusing retry logic and prompt infrastructure, and introduces a `callAI` function that returns raw output (instead of pre-cleaned messages) to support structured PR responses.

Reviewed-on: #2

src/types.ts

@@ -19,3 +19,13 @@ export interface ProjectContext {
   recentCommits: string[];
   diff: string;
 }
+
+export interface PRContext {
+  readme: string | null;
+  packageDescription: string | null;
+  structure: string | null;
+  branchName: string;
+  baseBranch: string;
+  branchCommits: string[];
+  diff: string;
+}