[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>

rust-toolchain.toml

@@ -0,0 +1,11 @@
+# SPDX-FileCopyrightText: 2026 Stan Grams <sjg@haxx.space>
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# Pins the Rust toolchain for reproducible builds. Keep in sync with the
+# SDK image (container/Containerfile, ARG RUST_VERSION). rustup honours this
+# automatically for local builds outside the SDK container.
+
+[toolchain]
+channel = "1.97.1"
+components = ["rustfmt", "clippy"]