feat: add gateway client with env-driven assembly

Includes config aggregation for multi-source env keys, from_env and
from_settings factories with explicit shared-backend injection,
gather_bounded, top-level exports, tightened import-linter layers with
the gate removed from the Makefile, and the finalized .env.example.

Makefile

@@ -8,12 +8,9 @@ install:
 test:
 	conda run -n $(ENV) pytest tests/ --cov=src/polygateway --cov-report=term-missing
 
-# lint-imports 在 M1 模块落地前门控跳过（契约见 pyproject.toml [tool.importlinter]）
 lint:
 	conda run -n $(ENV) ruff check src/ tests/ --fix
-	@conda run -n $(ENV) python -c "import polygateway.ports" 2>/dev/null \
-	  && conda run -n $(ENV) lint-imports \
-	  || echo "import-linter: M1 模块未创建，跳过（契约已在 pyproject.toml 声明）"
+	conda run -n $(ENV) lint-imports
 
 format:
 	conda run -n $(ENV) ruff format src/ tests/
@@ -21,9 +18,7 @@ format:
 check:
 	conda run -n $(ENV) ruff format --check src/ tests/
 	conda run -n $(ENV) ruff check src/ tests/
-	@conda run -n $(ENV) python -c "import polygateway.ports" 2>/dev/null \
-	  && conda run -n $(ENV) lint-imports \
-	  || echo "import-linter: M1 模块未创建，跳过（契约已在 pyproject.toml 声明）"
+	conda run -n $(ENV) lint-imports
 
 ci: check test