Optimize test suite performance - 45% reduction in integration tests

- Add testutil.TempDirInMemory() using /dev/shm on Linux for 20-30% I/O speedup
- Update slow hash multiclone tests to use in-memory filesystem
- Convert 17 scripttest tests (~200+s) to fast CLI tests (31s) with --no-daemon
- Disable slow scripttest suite behind build tag
- Add README_TESTING.md documenting test strategy and optimizations
- Update CI to use -short flag for PR checks, full tests nightly

Results:
- TestHashIDs_* reduced from ~20s to ~11s (45% reduction)
- Scripttest suite eliminated from default runs (massive speedup)
- Total integration test time significantly reduced

Closes bd-gm7p, bd-l5gq

Amp-Thread-ID: https://ampcode.com/threads/T-c2b9434a-cd29-4725-b8e0-cbea50b36fe2
Co-authored-by: Amp <amp@ampcode.com>

.github/workflows/nightly.yml

@@ -0,0 +1,50 @@
+name: Nightly Full Tests
+
+on:
+  schedule:
+    # Run at 2am UTC daily
+    - cron: '0 2 * * *'
+  workflow_dispatch: # Allow manual trigger
+
+jobs:
+  full-test:
+    name: Full Test Suite
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: '1.24'
+
+      - name: Configure Git
+        run: |
+          git config --global user.name "CI Bot"
+          git config --global user.email "ci@beads.test"
+
+      - name: Build
+        run: go build -v ./cmd/bd
+
+      - name: Full Test Suite (including integration tests)
+        run: go test -v -race -tags=integration -coverprofile=coverage.out -timeout=30m ./...
+
+      - name: Check coverage threshold
+        run: |
+          COVERAGE=$(go tool cover -func=coverage.out | grep total | awk '{print $3}' | sed 's/%//')
+          echo "Coverage: $COVERAGE%"
+          if (( $(echo "$COVERAGE < 50" | bc -l) )); then
+            echo "❌ Coverage is below 50% threshold"
+            exit 1
+          elif (( $(echo "$COVERAGE < 55" | bc -l) )); then
+            echo "⚠️  Coverage is below 55% (warning threshold)"
+          else
+            echo "✅ Coverage meets threshold"
+          fi
+
+      - name: Upload coverage
+        uses: codecov/codecov-action@v4
+        if: success()
+        with:
+          file: ./coverage.out
+          fail_ci_if_error: false