refactor: reorganize into monorepo with separate subprojects

Structure webber into three independent subprojects:
- webber-api/: FastAPI backend server with all agent code
- webber-cli/: Standalone CLI client (renamed from cli/ to webber_cli/)
- webber-sandbox/: Test project for functional testing

Key changes:
- Each subproject has its own .venv (Python 3.12+)
- Added sandbox.sh for managing test project templates
- Created sandbox-templates/ with calculator-cli and empty starter
- Updated CI/CD for prefixed tags (api/v*, cli/v*)
- Added comprehensive AGENTS.md with operational instructions
- Added gitignore filtering to glob and grep tools
- Created pyproject.toml for each subproject

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

webber-sandbox/tests/__init__.py

@@ -0,0 +1 @@
+"""Calculator tests."""

webber-sandbox/tests/test_operations.py

@@ -0,0 +1,46 @@
+"""
+Tests for calculator operations.
+
+NOTE: Test coverage is intentionally incomplete for testing purposes.
+"""
+import pytest
+
+from calculator.operations import add, subtract, multiply
+
+
+class TestAdd:
+    """Tests for add operation."""
+
+    def test_add_positive_numbers(self):
+        assert add(2, 3) == 5
+
+    def test_add_negative_numbers(self):
+        assert add(-2, -3) == -5
+
+    # MISSING: test_add_zero, test_add_floats
+
+
+class TestSubtract:
+    """Tests for subtract operation."""
+
+    def test_subtract_positive(self):
+        assert subtract(5, 3) == 2
+
+    # MISSING: test_subtract_negative, test_subtract_resulting_negative
+
+
+class TestMultiply:
+    """Tests for multiply operation."""
+
+    def test_multiply_positive(self):
+        assert multiply(3, 4) == 12
+
+    # MISSING: test_multiply_by_zero, test_multiply_negative
+
+
+# MISSING: TestDivide class entirely!
+# - test_divide_positive
+# - test_divide_by_zero (should test error handling)
+# - test_divide_negative
+
+# MISSING: TestPower class entirely!