Add automated git traffic benchmark for Agent Mail

- Created tests/benchmarks/git_traffic.py with 50-issue workflow
- Measures git operations reduction: git-only vs Agent Mail
- Generates comparison reports with pass/fail (70% target)
- Results: 98.5% reduction (200 → 3 operations)
- Added documentation in tests/benchmarks/README.md

Closes bd-ic1m

Amp-Thread-ID: https://ampcode.com/threads/T-0907bb40-ffc9-4f09-843f-b26716ef9ce0
Co-authored-by: Amp <amp@ampcode.com>

tests/benchmarks/README.md

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+# Beads Benchmarks
+
+Automated benchmarks for measuring Beads performance and Agent Mail coordination efficiency.
+
+## Git Traffic Reduction Benchmark
+
+**File:** `git_traffic.py`
+
+### Purpose
+
+Measures the reduction in git operations (pulls, commits, pushes) when using Agent Mail for multi-agent coordination compared to pure git-based synchronization.
+
+### Usage
+
+```bash
+# Run with default settings (50 issues)
+python3 tests/benchmarks/git_traffic.py
+
+# Customize number of issues
+python3 tests/benchmarks/git_traffic.py -n 100
+
+# Verbose output
+python3 tests/benchmarks/git_traffic.py -v
+
+# Save report to file
+python3 tests/benchmarks/git_traffic.py -o report.md
+```
+
+### How It Works
+
+The benchmark compares two workflows:
+
+**Without Agent Mail (Git-only mode):**
+- Each issue update requires git pull + commit + push
+- Other agents pull to check for updates
+- Total: ~4 git operations per issue
+
+**With Agent Mail:**
+- Coordination via HTTP messages (no git operations)
+- Status updates, reservations, notifications via Agent Mail
+- Single batched commit/push at end of workflow
+- Total: 3 git operations for entire batch
+
+### Expected Results
+
+For 50 issues:
+- **Without Agent Mail:** ~200 git operations
+- **With Agent Mail:** 3 git operations
+- **Reduction:** ≥70% (typically 95-98%)
+
+### Exit Codes
+
+- `0`: Success - achieved ≥70% reduction
+- `1`: Failure - regression detected
+
+### Example Output
+
+```
+======================================================================
+SUMMARY
+======================================================================
+Without Agent Mail: 200 git operations
+With Agent Mail:    3 git operations
+Reduction:          98.5%
+Target:             70%
+Status:             ✅ PASS
+======================================================================
+```
+
+## Requirements
+
+- Python 3.7+
+- bd (beads) CLI installed
+- git
+- Agent Mail server (optional - falls back to simulation if unavailable)
+
+## CI Integration
+
+This benchmark can be used in CI to detect regressions in Agent Mail coordination efficiency:
+
+```bash
+python3 tests/benchmarks/git_traffic.py -n 50
+# Exits with status 1 if reduction < 70%
+```