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>

install.sh

@@ -14,14 +14,14 @@
 # Environment:
 #   KIGI_SHARE_DIR        install root (default: ~/.kigi)
 #   KIGI_UPDATE_BASE_URL  releases API base, GitHub/Gitea-shaped (default:
-#                         https://api.github.com/repos/ZacharyZhang-NY/Kigi-CLI/releases)
+#                         https://git.zacharyzhang.com/api/v1/repos/ZacharyZhang-NY/Kigi-CLI/releases)
 #
 # Fails fast on any error; never leaves a partial binary as the active kigi.
 
 set -eu
 
 REPO="ZacharyZhang-NY/Kigi-CLI"
-API_BASE="${KIGI_UPDATE_BASE_URL:-https://api.github.com/repos/${REPO}/releases}"
+API_BASE="${KIGI_UPDATE_BASE_URL:-https://git.zacharyzhang.com/api/v1/repos/${REPO}/releases}"
 KIGI_HOME="${KIGI_SHARE_DIR:-$HOME/.kigi}"
 
 err() {