refactor: move act/model and act/exprparser to actions-proto-go

Gitea needs the workflow model and the expression evaluator to parse
workflows and to build the task payload this runner consumes, so it had
to depend on gitea.com/gitea/runner for it. Both packages now live in
gitea.dev/actions-proto-go, the module both sides already share, and this
repository consumes them from there.

The GithubContext helpers that need a git checkout on disk (SetRef,
SetSha and SetRepositoryAndOwner) are not moved: they are runner only and
would drag a git client and the act context logger into the shared
module. They become functions of the new act/ghcontext package, together
with their tests.

act/common.CartesianProduct moved to the shared model package as well,
act/model was its only user. act/model/testdata/container-volumes is now
act/runner/testdata/container-volumes, its only user is runner_test.go.

The x-runner-uuid and x-runner-token header names come from
actions-proto-go too, so the runner and Gitea cannot drift apart.

Signed-off-by: Lunny Xiao <xiaolunwen@gmail.com>

act/runner/testdata/container-volumes/push.yml

@@ -0,0 +1,19 @@
+name: Job Container
+on: push
+
+jobs:
+  with-volumes:
+    runs-on: ubuntu-latest
+    container:
+      image: node:24-bookworm-slim
+      volumes:
+        - my_docker_volume:/path/to/volume
+        - /path/to/nonexist/directory
+        - /proc/sys/kernel/random/boot_id:/current/boot_id
+    steps:
+      - run: |
+          set -e
+          test -d /path/to/volume
+          test "$(cat /proc/sys/kernel/random/boot_id)" = "$(cat /current/boot_id)"
+          test -d /path/to/nonexist/directory
+