fix(ci): resolve rustup conflict in CI for segkit checks

The Ubuntu CI runners have a pre-installed ~/.rustup toolchain that
conflicts with the Nix-managed Rust from devbox. When devbox's cargo
invokes clippy/rustfmt, the system rustup intercepts and tries to
sync components, causing file rename errors.

Fixes:
- Remove system rustup in the Segkit CI job before running devbox
- Remove rustfmt/clippy from treefmt.toml — they are project-level
  tools that don't fit treefmt's per-file model and were triggering
  the same rustup conflict in the Lint job. Rust checks now run
  exclusively in the dedicated Segkit CLI CI job.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: abueide

.github/workflows/pr-checks.yml

@@ -35,6 +35,9 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
+      - name: Remove system Rust toolchain
+        run: rustup self uninstall -y || true
+
       - name: Install Devbox
         uses: jetify-com/devbox-install-action@v0.15.0
         with:

treefmt.toml

@@ -1,5 +1,9 @@
 # treefmt configuration
-# Validates shell scripts, GitHub workflows, process-compose configs, and Rust code
+# Validates shell scripts, GitHub workflows, and process-compose configs
+#
+# Note: Rust checks (rustfmt, clippy) are NOT included here because they are
+# project-level tools that don't work well with treefmt's per-file model.
+# Run them via: devbox run segkit:check
 #
 # Usage:
 #   treefmt                    # Run validators on all files
@@ -78,29 +82,3 @@ includes = [
   "examples/*/tests/*.yaml",
 ]
 excludes = []
-
-# Rust formatting via rustfmt
-[formatter.rustfmt]
-command = "sh"
-options = [
-  "-c",
-  '''
-  cargo fmt --manifest-path segkit/Cargo.toml -- --check "$@" 2>&1 || exit 1
-  ''',
-  "--"
-]
-includes = ["segkit/src/**/*.rs", "segkit/tests/**/*.rs"]
-excludes = []
-
-# Rust linting via clippy
-[formatter.clippy]
-command = "sh"
-options = [
-  "-c",
-  '''
-  cargo clippy --manifest-path segkit/Cargo.toml -- -D warnings 2>&1 || exit 1
-  ''',
-  "--"
-]
-includes = ["segkit/src/**/*.rs", "segkit/tests/**/*.rs"]
-excludes = []