feat: add explain, review, changelog, and suggest commands

- gai explain: plain-language diff explanation with pipe support
- gai review: AI code review with strict/normal/lenient modes
- gai changelog: generate user-facing changelog from commits,
  supports --from/--to ranges and -n count
- gai suggest: suggest branch names or commit type from diff
- All commands support pipe input and auto-stage on empty staging area

src/commands/changelog.ts

@@ -0,0 +1,81 @@
+import { loadConfig } from "../config";
+import { isGitRepo, getRecentCommits } from "../git";
+import { CHANGELOG_SYSTEM_PROMPT, buildChangelogPrompt } from "../prompt";
+import { callAI } from "../ai";
+import { BOLD, RED, DIM, RESET, CYAN } from "../terminal";
+import { isStdinTTY } from "../tty";
+import type { StreamCallbacks } from "../types";
+import type { ParsedArgs } from "../cli";
+
+export async function handleChangelog(args: ParsedArgs): Promise<number> {
+  const config = await loadConfig();
+
+  if (!config.apiKey) {
+    console.error(`\n  ${RED()}Error: API key not set. Run ${BOLD()}gai config${RESET()}${RED()} to configure.${RESET()}\n`);
+    return 1;
+  }
+
+  if (!(await isGitRepo())) {
+    console.error(`\n  ${RED()}Error: Not a git repository.${RESET()}\n`);
+    return 1;
+  }
+
+  const from = args.flags["from"] as string | undefined;
+  const to = args.flags["to"] as string | undefined;
+  const countFlag = args.flags["count"] as number | undefined;
+  const verbose = args.flags["verbose"] as boolean;
+
+  // Collect commits
+  let commits: string[];
+
+  if (from) {
+    // Range-based: get commits between from..to
+    const range = to ? `${from}..${to}` : `${from}..HEAD`;
+    try {
+      const result = await Bun.$`git log --oneline ${range}`.quiet().text();
+      commits = result.trim().split("\n").filter(Boolean);
+    } catch {
+      console.error(`\n  ${RED()}Error: Invalid range: ${range}${RESET()}\n`);
+      return 1;
+    }
+  } else {
+    // Count-based
+    const count = typeof countFlag === "number" ? countFlag : 20;
+    commits = await getRecentCommits(count);
+  }
+
+  if (commits.length === 0) {
+    console.log(`  ${DIM()}No commits found for the specified range.${RESET()}`);
+    return 0;
+  }
+
+  if (verbose) {
+    console.log(`  ${DIM()}Processing ${commits.length} commits${RESET()}`);
+    console.log(`  ${DIM()}Model: ${config.model} | API: ${config.apiBase}${RESET()}`);
+  }
+
+  const userPrompt = buildChangelogPrompt(commits, from, to);
+
+  const tty = isStdinTTY();
+  if (tty) {
+    console.log(`\n  ${BOLD()}${CYAN()}Generating changelog from ${commits.length} commits...${RESET()}\n`);
+  }
+
+  try {
+    const callbacks: StreamCallbacks = tty ? {
+      onToken: (token) => process.stdout.write(token),
+    } : undefined;
+
+    const result = await callAI(config, CHANGELOG_SYSTEM_PROMPT, userPrompt, callbacks);
+    if (callbacks) {
+      process.stdout.write("\n");
+    } else {
+      process.stdout.write(result + "\n");
+    }
+  } catch (err) {
+    console.error(`\n  ${RED()}AI request failed: ${err instanceof Error ? err.message : err}${RESET()}\n`);
+    return 1;
+  }
+
+  return 0;
+}