chore(migration): repoint every published link off GitHub

The canonical repo is git.zacharyzhang.com now, so every URL a user can
actually reach had to move with it.

Self-updater: `PRODUCTION_ENDPOINTS.update_base_url` targets Gitea's
`/api/v1/repos/.../releases`, which serves the same wire format GitHub's
releases API did — `Release`/asset deserialisation is unchanged. The alpha
channel requests `?per_page=30&limit=30` because GitHub reads the page size
from `per_page` and Gitea from `limit`, and each ignores the other's, so one
URL stays correct on either.

Installers: same API base, and the documented one-liners become
`https://kigicli.dev/install.{sh,ps1}`. That domain's Worker proxies both
files from Gitea raw `main`, so the published install command survives any
future forge move without being reprinted everywhere. install.ps1 also drops
the GitHub-specific `Accept: application/vnd.github+json`.

Everything else user-facing: `/feedback` and the shell's feedback fallback
point at the Gitea issue tracker, `/docs` at the Gitea-rendered README, and
web_fetch's User-Agent at kigicli.dev. SECURITY.md now routes reports to the
issue tracker with a warning to withhold exploitable detail, because Gitea
has no equivalent of GitHub's private vulnerability reporting. Comments
naming `.github/workflows/release.yml` name `.gitea/` now.

Tests assert against these constants rather than literal URLs, so they
follow automatically; the one URL-shape assertion (production endpoints are
https) still holds.

Co-Authored-By: Claude Opus 5 (1M context) <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 `.github/workflows/release.yml`.
+/// release-asset naming in `.gitea/workflows/release.yml`.
 pub fn host_triple() -> &'static str {
     if cfg!(all(target_os = "macos", target_arch = "aarch64")) {
         "aarch64-apple-darwin"