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>

.gitignore

@@ -64,3 +64,14 @@ Thumbs.db
 # Project specific
 *.db
 *.sqlite3
+
+# Monorepo - subproject venvs (explicit for clarity)
+webber-api/.venv/
+webber-cli/.venv/
+webber-sandbox/.venv/
+
+# Sandbox marker file
+webber-sandbox/.current_template
+
+# Ruff cache
+.ruff_cache/