ci/eval: name the base branch in the performance comparison summary

The performance comparison summary previously stated it compared against
"its pull request base branch (e.g., 'master')" regardless of the actual
base branch.

Plumb the base branch classification computed in prepare.js through the
eval workflow to the `compare` derivation so the summary names the real
base branch (e.g. "staging-25.11"). The local `eval.full` helper defaults
to "master", matching its touched-files convention.

Assisted-by: claude-code with claude-opus-4-8[1m]-high

.github/workflows/eval.yml

@@ -15,6 +15,9 @@ on:
       targetSha:
         required: true
         type: string
+      baseBranch:
+        required: true
+        type: string
       systems:
         required: true
         type: string
@@ -291,6 +294,7 @@ jobs:
       - name: Compare against the target branch
         env:
           TARGET_SHA: ${{ inputs.mergedSha }}
+          BASE_BRANCH: ${{ fromJSON(inputs.baseBranch).branch }}
         run: |
           git -C nixpkgs/trusted diff --name-only "$TARGET_SHA" \
             | jq --raw-input --slurp 'split("\n")[:-1]' > touched-files.json
@@ -299,6 +303,7 @@ jobs:
           nix-build nixpkgs/trusted/ci --arg nixpkgs ./nixpkgs/trusted-pinned -A eval.compare \
             --arg combinedDir ./combined \
             --arg touchedFilesJson ./touched-files.json \
+            --argstr baseBranch "$BASE_BRANCH" \
             --out-link comparison
 
           cat comparison/step-summary.md >> "$GITHUB_STEP_SUMMARY"

.github/workflows/merge-group.yml

@@ -97,6 +97,7 @@ jobs:
       artifact-prefix: ${{ inputs.artifact-prefix }}
       mergedSha: ${{ needs.prepare.outputs.mergedSha }}
       targetSha: ${{ needs.prepare.outputs.targetSha }}
+      baseBranch: ${{ needs.prepare.outputs.baseBranch }}
       systems: ${{ needs.prepare.outputs.systems }}
 
   build:

.github/workflows/pull-request-target.yml

@@ -110,6 +110,7 @@ jobs:
       mergedSha: ${{ needs.prepare.outputs.mergedSha }}
       headSha: ${{ github.event.pull_request.head.sha }}
       targetSha: ${{ needs.prepare.outputs.targetSha }}
+      baseBranch: ${{ needs.prepare.outputs.baseBranch }}
       systems: ${{ needs.prepare.outputs.systems }}
       testVersions: ${{ contains(fromJSON(needs.prepare.outputs.touched), 'pinned') && !contains(fromJSON(needs.prepare.outputs.headBranch).type, 'development') }}