fix(ci): switch runner label to self-hosted, extend matrix to 3.13

The build steps now declare 'runs-on: self-hosted' (default label
advertised by an act_runner registered with Docker mode). Previously
they used 'ubuntu-22.04' which would not match any default runner and
would leave every job in 'no runner available'.

ci.yml matrix extended to ['3.10','3.11','3.12','3.13'] for forward
coverage on Python 3.13 (current stable as of 2026).

docs/development.md 'CI/CD - Gitea Actions' section also corrected:
- the egress workaround now uses the actual Gitea 1.26 key
  [actions].DEFAULT_ACTIONS_URL (not ENABLE_GITHUB_ACTIONS_MIRROR)
- the act_runner docker command uses --rm -it for the first run to
  surface registration errors in the terminal before daemonization
- runner-scoping note clarifies global vs repo-scoped token URLs

Verified by ruff_exit=0 and pytest 66/66 still green locally.

.gitea/workflows/ci.yml

@@ -13,7 +13,7 @@ concurrency:
 jobs:
   ruff:
     name: lint (ruff)
-    runs-on: ubuntu-22.04
+    runs-on: self-hosted
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -33,11 +33,11 @@ jobs:
 
   pytest:
     name: pytest (py${{ matrix.python-version }})
-    runs-on: ubuntu-22.04
+    runs-on: self-hosted
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.10", "3.11", "3.12"]
+        python-version: ["3.10", "3.11", "3.12", "3.13"]
     steps:
       - name: Checkout
         uses: actions/checkout@v4