feat(mcp): Consolidate context tools into unified 'context' tool

Merge set_context, where_am_i, and init into a single 'context' tool
with action parameter (set, show, init).

- set: Set the workspace root directory (default when workspace_root provided)
- show: Show current workspace context (default when no args)
- init: Initialize beads in the current workspace directory

This reduces tool count from 3 to 1 while maintaining all functionality.

Closes beads-eub

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

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

integrations/beads-mcp/tests/test_workspace_auto_detect.py

@@ -107,7 +107,7 @@ async def test_get_client_no_workspace_found():
                 # Verify error message is helpful
                 error_msg = str(exc_info.value)
                 assert "No beads workspace found" in error_msg
-                assert "set_context" in error_msg
+                assert "context" in error_msg
                 assert ".beads/" in error_msg
     finally:
         current_workspace.reset(token)