feat: add amcs-cli — MCP tool CLI and stdio bridge

- cmd/amcs-cli: new CLI tool for human/AI MCP tool access
- amcs-cli tools: list all tools from remote MCP server
- amcs-cli call <tool> --arg k=v: invoke a tool, print JSON/YAML result
- amcs-cli stdio: stdio→HTTP MCP bridge for AI clients
- Config: ~/.config/amcs/config.yaml, AMCS_URL/AMCS_TOKEN env vars, --server/--token flags
- Token never logged in errors
- Makefile: add build-cli target

Closes #18

Makefile

@@ -78,3 +78,8 @@ check-schema-drift:
 		exit 1; \
 	fi; \
 	rm -f $$tmpfile
+
+.PHONY: build-cli
+build-cli:
+	@mkdir -p $(BIN_DIR)
+	go build -o $(BIN_DIR)/amcs-cli ./cmd/amcs-cli