Fix bd-8zf2: MCP server auto-detects workspace from CWD

- Add _find_beads_db_in_tree() to walk up looking for .beads/*.db
- Update _get_client() to auto-detect when workspace not set
- Matches CLI behavior (no manual set_context needed after restart)
- Add 8 comprehensive tests for auto-detection
- Update existing tests to mock auto-detection in error cases

Fixes silent failures after Amp restart.

Amp-Thread-ID: https://ampcode.com/threads/T-c47f524d-c101-40d5-839a-659f52b9be48
Co-authored-by: Amp <amp@ampcode.com>

integrations/beads-mcp/tests/test_daemon_health_check.py

@@ -299,5 +299,7 @@ async def test_get_client_no_workspace_error():
     tools.current_workspace.set(None)
     
     with patch.dict('os.environ', {}, clear=True):
-        with pytest.raises(BdError, match="No workspace set"):
-            await _get_client()
+        # Mock auto-detection to fail
+        with patch("beads_mcp.tools._find_beads_db_in_tree", return_value=None):
+            with pytest.raises(BdError, match="No beads workspace found"):
+                await _get_client()