Move releases, self-update and links to ELY-Open-Source on GitHub

- kigi-env: update_base_url defaults to
  api.github.com/repos/ELY-Open-Source/Kigi-CLI/releases.
- install.sh / install.ps1: resolve releases from the same API.
- /feedback and the no-session feedback fallback open GitHub issues/new;
  /docs opens the README on GitHub.
- SECURITY, NOTICE, AGENTS, CONTRIBUTING: GitHub repo, issues and fork-based
  pull requests replace git.zacharyzhang.com and AGit.

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

crates/codegen/kigi-update/tests/common/mod.rs

@@ -114,7 +114,7 @@ pub fn host_platform() -> String {
 }
 
 /// Host Rust target triple, matching `auto_update::target_triple()` and the
-/// release-asset naming in `.gitea/workflows/release.yml`.
+/// release-asset naming in `.github/workflows/release.yml`.
 pub fn host_triple() -> &'static str {
     if cfg!(all(target_os = "macos", target_arch = "aarch64")) {
         "aarch64-apple-darwin"