[chore](trx-rs): shared SDK image for CI and developers

Rework container/ from a host-executor act_runner image into a single
"SDK" build image used everywhere: as the CI job container (Docker
executor) and by developers locally / via .devcontainer. It bakes in a
pinned Rust toolchain and all build dependencies, so CI and every
developer share the exact same rustc/clippy.

- container/Containerfile: SDK image (Debian + deps + pinned Rust + Node).
- rust-toolchain.toml: pin the toolchain to match the image; also ends the
  "CI clippy newer than local" version skew.
- .gitea/workflows/ci.yml: lint/test run inside the SDK image via
  `container:`; reuse returns to fsfe/reuse-action (Docker executor runs
  it as a sibling container, so nothing REUSE-related is baked in).
- .devcontainer/devcontainer.json: dev use of the same image.
- container/runner-config.example.yaml: Docker-executor runner config for
  the CI VM, capped for a 2-thread budget.
- Drop the obsolete host-executor entrypoint/config/Quadlet units.

Assisted-By: Claude Code (claude-opus-4)
Claude-Session: https://claude.ai/code/session_01NFpGtGTWUEYXLwZeZs2RAV
Signed-off-by: Stan Grams <sjg@haxx.space>

container/runner-config.example.yaml

@@ -0,0 +1,33 @@
+# SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# Example act_runner config for the Docker-executor runner that lives in the
+# CI VM. This is NOT the SDK image — it configures the runner that launches
+# per-job containers (including the trx-rs SDK image referenced by the
+# workflow's `container:` key). Copy to the VM and pass with
+# `act_runner daemon --config`.
+
+log:
+  level: info
+
+runner:
+  file: .runner
+  # One concurrent job. With one runner per project on a 2-vCPU VM this keeps
+  # total CI usage at ~2 threads.
+  capacity: 1
+  timeout: 3h
+  # Docker executor: no ":host" suffix. Maps runs-on labels to base images
+  # (the workflow overrides these per job via `container:`).
+  labels:
+    - "ubuntu-latest:docker://catthehacker/ubuntu:act-latest"
+
+cache:
+  enabled: true
+
+container:
+  # Cap every job container's CPU so CI stays within the 2-thread budget even
+  # if capacity is raised later.
+  options: "--cpus=2"
+  # Reuse the host VM's Docker network for the built-in cache/artifact server.
+  network: "host"