Keep Rust PRs green with a minimal CI gate

Add a focused GitHub Actions workflow for pull requests into main plus
manual dispatch. The workflow checks workspace formatting and runs the
rusty-claude-cli crate tests so we get a real signal on the active Rust
surface without widening scope into a full matrix.

Because the workspace was not rustfmt-clean, include the formatting-only
updates needed for the new fmt gate to pass immediately.

Constraint: Keep scope to a fast, low-noise Rust PR gate
Constraint: CI should validate formatting and rusty-claude-cli without expanding to full workspace coverage
Rejected: Full workspace test or clippy matrix | too broad for the one-hour shipping window
Rejected: Add fmt CI without reformatting the workspace | the new gate would fail on arrival
Confidence: high
Scope-risk: narrow
Directive: Keep this workflow focused unless release requirements justify broader coverage
Tested: cargo fmt --all -- --check
Tested: cargo test -p rusty-claude-cli
Tested: YAML parse of .github/workflows/rust-ci.yml via python3 + PyYAML
Not-tested: End-to-end execution on GitHub-hosted runners

.github/workflows/rust-ci.yml

@@ -0,0 +1,48 @@
+name: Rust CI
+
+on:
+  pull_request:
+    branches:
+      - main
+    paths:
+      - .github/workflows/rust-ci.yml
+      - rust/**
+  workflow_dispatch:
+
+concurrency:
+  group: rust-ci-${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+defaults:
+  run:
+    working-directory: rust
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  fmt:
+    name: cargo fmt
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt
+      - uses: Swatinem/rust-cache@v2
+        with:
+          workspaces: rust -> target
+      - name: Check formatting
+        run: cargo fmt --all -- --check
+
+  test-rusty-claude-cli:
+    name: cargo test -p rusty-claude-cli
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@v2
+        with:
+          workspaces: rust -> target
+      - name: Run crate tests
+        run: cargo test -p rusty-claude-cli