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/prompt.ts

@@ -1,4 +1,4 @@
-import type { ProjectContext } from "./types";
+import type { PRContext, ProjectContext } from "./types";
 
 export const SYSTEM_PROMPT = `You are an expert at writing concise, meaningful git commit messages following the Conventional Commits specification.
 
@@ -54,3 +54,65 @@ export function buildPrompt(context: ProjectContext): string {
 
   return parts.join("\n");
 }
+
+export const PR_SYSTEM_PROMPT = `You are an expert at writing clear, concise pull request titles and descriptions.
+
+Format:
+<pr title>
+<blank line>
+<pr body>
+
+Rules:
+1. Title must be under 72 characters, in imperative mood
+2. Follow the Conventional Commits style for the title (e.g., "feat(api): add user authentication")
+3. Body should be 2-3 sentences in plain text explaining WHAT was changed and WHY
+4. Be specific — avoid vague messages
+5. Match the language and style of recent commits if provided
+6. If the branch name hints at the type (e.g., "feat/..." or "fix/..."), reflect that in the title
+7. Output ONLY the PR text — no markdown, no code blocks, no prefixes`;
+
+export function buildPRPrompt(context: PRContext): string {
+  const parts: string[] = [];
+
+  if (
+    context.packageDescription ||
+    context.readme ||
+    context.structure
+  ) {
+    parts.push("## Project Context");
+    if (context.packageDescription) {
+      parts.push(`Description: ${context.packageDescription}`);
+    }
+    if (context.structure) {
+      parts.push(`Structure: ${context.structure}`);
+    }
+    if (context.readme) {
+      parts.push(`README:\n${context.readme}`);
+    }
+    parts.push("");
+  }
+
+  parts.push("## Branch Info");
+  parts.push(`Branch: ${context.branchName}`);
+  parts.push(`Target base: ${context.baseBranch}`);
+  parts.push("");
+
+  if (context.branchCommits.length > 0) {
+    parts.push("## Commits on This Branch");
+    for (const c of context.branchCommits) {
+      parts.push(c);
+    }
+    parts.push("");
+  }
+
+  parts.push("## Changes (diff from base)");
+  parts.push("```diff");
+  parts.push(context.diff);
+  parts.push("```");
+  parts.push("");
+  parts.push(
+    "Generate a pull request title and brief body for the above changes.",
+  );
+
+  return parts.join("\n");
+}