Add Gitea Actions workflows, CI config, and docs

.gitea/template

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+# Gitea Template Variable Expansion
+# ----------------------------------
+# When a new repo is created from this template, Gitea will replace
+# template variables (e.g. ${REPO_NAME}) in files matching the globs below.
+#
+# Supported variables (Gitea ≥ 1.20):
+#   REPO_NAME          – name of the new repository
+#   REPO_DESCRIPTION   – description entered during creation
+#   TEMPLATE_NAME      – name of this template repository
+#   YEAR               – current four-digit year
+#   MONTH              – current zero-padded month (01-12)
+#   DAY                – current zero-padded day (01-31)
+#   MONTH_ENGLISH      – current month in English (January, February, …)
+#
+# Glob patterns — one per line. Files matching these will have variables expanded.
+
+README.md
+docs/*.md
+.ci/config.env
+SECURITY.md
+CONTRIBUTING.md
+CODE_OF_CONDUCT.md
+LICENSE
+pull_request_template.md