Fix skill eval change detection to compare full PR diff against base

The previous logic used 'git diff-tree -r HEAD' which only compares the
latest commit against its parent. On PRs with multiple commits, this
misses skill changes from earlier commits and incorrectly skips evals.

Now uses the PR base SHA (github.event.pull_request.base.sha) with
three-dot diff to detect ALL skill file changes across the entire PR.
Also uses fetch-depth: 0 so the base SHA is available.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ronniegeraghty

.github/workflows/skill-eval.yml

@@ -61,14 +61,23 @@ jobs:
     steps:
       - uses: actions/checkout@v4
         with:
-          fetch-depth: 2
+          fetch-depth: 0
 
       - name: Check if skill files changed
         id: changes
         run: |
           if [ "${{ github.event_name }}" = "workflow_dispatch" ]; then
             echo "SHOULD_RUN=true" >> $GITHUB_OUTPUT
-          elif git diff-tree --no-commit-id --name-only -r HEAD | grep -q "^\.github/skills/"; then
+          elif [ "${{ github.event_name }}" = "pull_request" ]; then
+            # Compare PR head against base branch to detect all changes in the PR
+            BASE_SHA="${{ github.event.pull_request.base.sha }}"
+            if git diff --name-only "$BASE_SHA"...HEAD | grep -q "^\.github/skills/"; then
+              echo "SHOULD_RUN=true" >> $GITHUB_OUTPUT
+            else
+              echo "SHOULD_RUN=false" >> $GITHUB_OUTPUT
+              echo "No skill file changes detected in PR — skipping evals"
+            fi
+          elif git diff --name-only HEAD~1 HEAD | grep -q "^\.github/skills/"; then
             echo "SHOULD_RUN=true" >> $GITHUB_OUTPUT
           else
             echo "SHOULD_RUN=false" >> $GITHUB_OUTPUT