refactor: Remove legacy MCP Agent Mail integration (bd-6gd)

Remove the external MCP Agent Mail server integration that required
running a separate HTTP server and configuring environment variables.

The native `bd mail` system (stored as git-synced issues) remains
unchanged and is the recommended approach for inter-agent messaging.

Files removed:
- cmd/bd/message.go - Legacy `bd message` command
- integrations/beads-mcp/src/beads_mcp/mail.py, mail_tools.py
- lib/beads_mail_adapter.py - Python adapter library
- examples/go-agent/ - Agent Mail-focused example
- examples/python-agent/agent_with_mail.py, AGENT_MAIL_EXAMPLE.md
- docs/AGENT_MAIL*.md, docs/adr/002-agent-mail-integration.md
- tests/integration/test_agent_race.py, test_mail_failures.py, etc.
- tests/benchmarks/ - Agent Mail benchmarks

Updated documentation to remove Agent Mail references while keeping
native `bd mail` documentation intact.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/integration/README.md

@@ -2,102 +2,18 @@
 
 This directory contains integration tests for bd (beads) that test end-to-end functionality.
 
-## Tests
-
-### test_agent_race.py
-
-Multi-agent race condition test that validates collision prevention with Agent Mail.
-
-**What it tests:**
-- Multiple agents simultaneously attempting to claim the same issue
-- WITH Agent Mail: Only one agent succeeds (via reservation)
-- WITHOUT Agent Mail: Multiple agents may succeed (collision)
-- Verification via JSONL that no duplicate claims occur
-
-### test_mail_failures.py
-
-Agent Mail server failure scenarios test that validates graceful degradation.
-
-**What it tests:**
-- Server never started (connection refused)
-- Server crashes during operation  
-- Network partition (timeout)
-- Server returns 500 errors
-- Invalid bearer token (401)
-- Malformed JSON responses
-- JSONL consistency under multiple failures
-
-**Performance:**
-- Uses `--no-daemon` flag for fast tests (~33s total)
-- 1s HTTP timeouts for quick failure detection
-- Mock HTTP server avoids real network calls
-
-### test_reservation_ttl.py
-
-Reservation TTL and expiration test that validates time-based reservation behavior.
-
-**What it tests:**
-- Short TTL reservations (30s)
-- Reservation blocking verification (agent2 cannot claim while agent1 holds reservation)
-- Auto-release after expiration (expired reservations become available)
-- Renewal/heartbeat mechanism (re-reserving extends expiration)
-
-**Performance:**
-- Uses `--no-daemon` flag for fast tests
-- 30s TTL for expiration tests (includes wait time)
-- Total test time: ~57s (includes 30s+ waiting for expiration)
-- Mock HTTP server with full TTL management
-
 ## Prerequisites
 
 - bd installed: `go install github.com/steveyegge/beads/cmd/bd@latest`
-- Agent Mail server running (optional, for full test suite):
-  ```bash
-  cd ~/src/mcp_agent_mail
-  source .venv/bin/activate
-  uv run python -m mcp_agent_mail.cli serve-http
-  ```
+- Python 3.7+ for Python-based tests
 
 ## Running Tests
 
-**Run test_agent_race.py:**
 ```bash
-python3 tests/integration/test_agent_race.py
+# Run all integration tests
+python3 -m pytest tests/integration/
 ```
 
-**Run test_mail_failures.py:**
-```bash
-python3 tests/integration/test_mail_failures.py
-```
-
-**Run test_reservation_ttl.py:**
-```bash
-python3 tests/integration/test_reservation_ttl.py
-```
-
-**Run all integration tests:**
-```bash
-python3 tests/integration/test_agent_race.py
-python3 tests/integration/test_mail_failures.py
-python3 tests/integration/test_reservation_ttl.py
-```
-
-## Expected Results
-
-### test_agent_race.py
-- **WITH Agent Mail running:** Test 1 passes (only 1 claim), Test 2 shows collision, Test 3 passes
-- **WITHOUT Agent Mail running:** All tests demonstrate collision (expected behavior without reservation system)
-
-### test_mail_failures.py
-- All 7 tests should pass in ~30-35 seconds
-- Each test validates graceful degradation to Beads-only mode
-- JSONL remains consistent across all failure scenarios
-
-### test_reservation_ttl.py
-- All 4 tests should pass in ~57 seconds
-- Tests verify TTL-based reservation expiration and renewal
-- Includes 30s+ wait time to validate actual expiration behavior
-
 ## Adding New Tests
 
 Integration tests should: