chore: restore missing metrics and CI optimizations

This PR restores critical fixes for repository metrics and CI cost optimizations that were identified as missing from the filesystem despite being marked as completed in previous tasks.

### Changes:
- **Metrics Accuracy**: Re-implemented 7-day fixed window and search-based sampling in `throughput.ts`, `latency.ts`, and `user_touches.ts` to resolve reporting anomalies during batch operations.
- **Spend Tracking**: Implemented pagination in `actions_spend.ts` to ensure all workflow runs within the 7-day window are captured, avoiding undercounting.
- **CI Cost Optimization**: Replaced all instances of `macos-latest-large` with standard `macos-latest` runners in `ci.yml`, `chained_e2e.yml`, and `deflake.yml`.
- **Matrix Reduction**: Reduced the Mac test matrix in `ci.yml` to Node 20.x only, significantly reducing redundant compute spend.
- **Task Ledger**: Updated `lessons-learned.md` to document the logic divergence and its resolution (BT-63).

These changes ensure the repository metrics are reliable and that CI costs remain under control.

.github/workflows/ci.yml

@@ -231,7 +231,7 @@ jobs:
 
   test_mac:
     name: 'Test (Mac) - ${{ matrix.node-version }}, ${{ matrix.shard }}'
-    runs-on: 'macos-latest-large'
+    runs-on: 'macos-latest'
     needs:
       - 'merge_queue_skipper'
     if: "github.repository == 'google-gemini/gemini-cli' && needs.merge_queue_skipper.outputs.skip == 'false'"
@@ -244,8 +244,6 @@ jobs:
       matrix:
         node-version:
           - '20.x'
-          - '22.x'
-          - '24.x'
         shard:
           - 'cli'
           - 'others'