Consolidate Gitea CI into one event-routed workflow

AGENTS.md

@@ -225,13 +225,16 @@ cached.
 - Do not modify files in `archive/` — reference only.
 - Build needs SDL3 + SDL3_ttf/image/mixer/net + GTest installed; CI uses
   Gitea Actions on a self-hosted host runner (`build-server`, no docker).
-  The workflow lives in `.gitea/workflows/build.yml` and is **manual-only**
-  (`workflow_dispatch`); pick the branch when dispatching. It runs
+  The single workflow `.gitea/workflows/build.yml` has two triggers:
+  **`workflow_dispatch`** always builds the `dev` branch and publishes the dev
+  cache/manifest, and a **tag push** (`tags: ['*']`) builds the tag and
+  publishes the release cache/manifest. The event name routes the steps
+  (`github.event_name`). It runs
   `nix build -L .#checks.<system>.default` (build + the offline test suite;
   `checks` is keyed per-system, so the workflow resolves the current system
   via `nix eval --impure --expr 'builtins.currentSystem'`). The build bakes
-  in a per-branch `APP_VERSION` — the current git tag on `main`, the current
-  commit hash on `dev`. The flake reads `APP_VERSION` from the environment via
+  in an `APP_VERSION` — the short commit hash on a manual dev run, the current
+  git tag on a tag push. The flake reads `APP_VERSION` from the environment via
   `builtins.getEnv`, so the build must pass `--impure` (pure evals get the
   package defaults). The live portal tests run from `nix develop` when the
   `RWT_LIVE_*` secrets are set and need `RWT_LIVE_ACCOUNT_EMAIL`,