Return the forge to GitHub: releases, self-update, links, CI

The GitHub account is restored, so github.com/ZacharyZhang-NY/Kigi-CLI is
the canonical repo again. Reverse the 2026-08 Gitea migration:

- .github/workflows/{release,warm-cache}.yml: restore the GitHub-hosted
  pipeline dropped in 3e1652c, unchanged — hosted runners, rust-cache,
  softprops release with every asset attached atomically.
- .gitea/workflows/release.yml: retire the Gitea edition (the runners are
  being deregistered; git.zacharyzhang.com keeps the v0.1.12/v0.1.13
  releases for old clients).
- kigi-env: update_base_url now defaults to the GitHub Releases API. The
  updater already speaks GitHub wire format and sends a User-Agent; v0.1.14
  is published to BOTH forges with identical assets so pre-0.1.14 installs
  polling Gitea self-update onto the GitHub channel.
- install.sh / install.ps1: resolve releases from api.github.com.
- feedback, /docs, SECURITY.md, frontend worker + site: repo, issues and
  raw-file links point at github.com / raw.githubusercontent.com.
- docs/RELEASE.md: GitHub edition of the checklist; Gitea runner sections
  retired.
- version: 0.1.13 -> 0.1.14 (the cross-forge transition release).

Claude-Session: https://claude.ai/code/session_01A6RcwHhhsFYYrNdQB4ASuP

crates/codegen/kigi-env/src/lib.rs

@@ -21,7 +21,7 @@ pub struct KigiEndpoints {
 pub const PRODUCTION_ENDPOINTS: KigiEndpoints = KigiEndpoints {
     coding_api_base_url: "https://api.kimi.com/coding/v1",
     oauth_host: "https://auth.kimi.com",
-    update_base_url: "https://git.zacharyzhang.com/api/v1/repos/ZacharyZhang-NY/Kigi-CLI/releases",
+    update_base_url: "https://api.github.com/repos/ZacharyZhang-NY/Kigi-CLI/releases",
     upgrade_page_url: "https://www.kimi.com/code/",
 };