test(bf-25hf0): capture grep-corpus baseline metrics

- Record corpus metrics: 1000 PDF files, 6.6 MB total
- Commit SHA: 77eeaecc
- Note: grep subcommand not yet implemented (blocked on 7.8.x beads)
- Timing throughput metrics unavailable until implementation complete

Acceptance criteria status:
- PASS: benches/baselines/grep-corpus.json exists and is valid JSON
- PASS: JSON contains required fields (commit_sha, timestamp, corpus_size)
- PASS: Corpus validation passes (1000 files confirmed)
- WARN: Grep implementation not complete, timing metrics deferred
- PASS: Timestamp is current, commit hash matches HEAD

benches/baselines/grep-corpus.json

@@ -0,0 +1,7 @@
+{
+  "commit_sha": "77eeaecc",
+  "timestamp": "2026-07-06T16:06:55+00:00",
+  "corpus_size": 1000,
+  "bytes_total": 6870672,
+  "notes": "Baseline captured before grep implementation (blocked on 7.8.x beads). Corpus metrics only - timing throughput not yet available. Grep subcommand not yet implemented."
+}