Add Python 3.13 to CI matrix and optimize linting steps

Adds Python 3.13 to the test matrix. Optimizes CI by running linting
and type checking only on Python 3.12 instead of all versions.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.10", "3.11", "3.12"]
+        python-version: ["3.10", "3.11", "3.12", "3.13"]
 
     steps:
       - uses: actions/checkout@v4
@@ -36,11 +36,13 @@ jobs:
           pip install -e ".[dev]"
 
       - name: Run linting
+        if: matrix.python-version == '3.12'
         run: |
           ruff check .
           ruff format --check .
 
       - name: Run type checking
+        if: matrix.python-version == '3.12'
         run: |
           mypy headroom --ignore-missing-imports