fix: enable AUTOFIX_PR feature flag by default in build

`docs/jira/AUTOFIX-PR-001.md:134` documents that the AUTOFIX_PR flag
"should be added to DEFAULT_BUILD_FEATURES" but the actual addition
to scripts/defines.ts was never made. Result: every build/dev produced
an /autofix-pr command whose isEnabled() returns false (because
src/commands/autofix-pr/index.ts:9 gates on `feature('AUTOFIX_PR')`),
so the command never appeared in /help and was effectively dead code.

The flag is fork-introduced (0 references in upstream/main) so adding
it here doesn't conflict with upstream policy. Other feature() calls
the PR added (KAIROS_GITHUB_WEBHOOKS, etc.) are upstream-owned flags
where we shouldn't override the upstream default.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: unraid

scripts/defines.ts

@@ -92,4 +92,6 @@ export const DEFAULT_BUILD_FEATURES = [
   // 'TEAMMEM',                  // 已禁用：依赖 COORDINATOR_MODE，邮箱文件无限增长
   // SSH Remote
   'SSH_REMOTE', // SSH 远程连接，本地 REPL + 远端工具执行
+  // Autofix PR
+  'AUTOFIX_PR', // /autofix-pr 命令（fork 引入；docs/jira/AUTOFIX-PR-001.md 承诺默认开启）
 ] as const