fix: align PR checks workflow environment configuration

Fix critical environment mismatch in PR checks workflow:
- setup-pixi environments: quality -> ci
- Ensures 'ci' environment is installed when workflow runs 'pixi run -e ci'

Root cause of "format-check: command not found" (exit code 127):
- setup-pixi was installing 'quality' environment
- Workflow was trying to run tasks in 'ci' environment
- 'ci' environment was never installed, so format-check task unavailable

This resolves the core pixi environment alignment issue.

Iteration: 5/10 for pixi_command_not_found (environment mismatch)
Job: PR Quality Gate, Coverage Check
Error: Process completed with exit code 127

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: MementoRC

.github/workflows/pr-checks.yml

@@ -143,12 +143,12 @@ jobs:
       uses: prefix-dev/[email protected]
       with:
         manifest-path: pyproject.toml
-        environments: quality
+        environments: ci
         cache: false
 
     - name: Install project in editable mode
       shell: bash
-      run: pixi run -e quality install-editable
+      run: pixi run -e ci install-editable
 
     - name: Generate coverage report
       shell: bash