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

The GitHub account is gone again, so git.zacharyzhang.com/ZacharyZhang-NY/Kigi-CLI
is the canonical repo. Reverse f7a4cb6:

- .gitea/workflows/release.yml: restore the Gitea edition retired in f7a4cb6
  (draft release, per-target upload, SHA256SUMS, then publish). The Windows
  leg runs on the windows-amd64 label the current runner advertises.
- .github/workflows/{release,warm-cache}.yml: drop them; they target hosted
  GitHub runners and softprops, and Gitea ignores them once .gitea exists.
- kigi-env: update_base_url defaults to the Gitea Releases API again.
- install.sh / install.ps1: resolve releases from git.zacharyzhang.com.
- feedback, /docs, SECURITY.md, frontend worker + site: repo, issues and
  raw-file links point at git.zacharyzhang.com.
- docs/RELEASE.md: Gitea edition of the checklist with the current runners.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

NOTICE

@@ -14,7 +14,7 @@ Modifications relative to the upstream work (Apache License 2.0 §4(b) notice):
   subscription-proxy endpoints.
 - Endpoints re-targeted at the Kimi Code subscription API
   (api.kimi.com/coding/v1), the Moonshot open platform (api.moonshot.cn/v1,
-  api.moonshot.ai/v1), and GitHub Releases for self-update.
+  api.moonshot.ai/v1), and Gitea Releases (git.zacharyzhang.com) for self-update.
 
 This product includes code ported from openai/codex and sst/opencode; see
 crates/codegen/kigi-tools/THIRD_PARTY_NOTICES.md for the license terms and