base prompt結合機能を追加

config.tomlの[prompt]でpositive/negativeのbase promptファイルを指定し、
MCP経由でLLMから渡されたプロンプトと結合してpipelineへ渡す機能を追加する。
プロンプトファイルは<root>/prompts/<file>に固定し、TOMLにはファイル名のみ
指定する。モデルパスと同じ方針でパストラバーサルを拒否する。

結合方式:
- positive: base_positive, user_positive の順でカンマ区切り
- negative: base_negative\nuser_negative の順で改行区切り
- baseが空の場合はuserプロンプトのみ（後方互換）
- [prompt]省略時はbase promptなし（後方互換）

結合場所はschema.pyのTextToImageRequest.create()に集約し、GuidedImageRequest
とInpaintImageRequestは内部でTextToImageRequest.create()を呼ぶため1箇所の
変更で3ツールすべてに適用される。MCP Tool入力にはbase promptを公開しない。

変更内容:
- config.py: PromptConfig dataclass、AppConfig.promptフィールド、
  _read_promptヘルパー、load_configの[prompt]読込・validation追加。
  positive_file/negative_fileはファイル名のみ受け付け、<root>/prompts/へ解決
- schema.py: _combineヘルパー、TextToImageRequest.create()に
  base_positive/base_negative引数追加、GuidedImageRequest.create()と
  InpaintImageRequest.create()から転送
- tools/generate_image.py: service.config.promptからbaseを取りcreate()へ渡す
- tools/generate_guided_image.py: 同上
- tools/inpaint_image.py: 同上
- tests/conftest.py: app_config fixtureにprompt=PromptConfig()追加
- tests/test_config.py: [prompt]の正常系・異常系テスト追加
  (省略時空、ファイル読込、パストラバーサル拒否、ファイル不存在、positiveのみ)
- tests/test_schema.py: 結合ロジックのテスト追加
  (base両方、positiveのみ、negativeのみ、base空、guided/inpaint転送)
- doc/config.md: §10 [prompt]セクション追記、設定項目一覧表・ディレクトリ構成・
  全設定例更新、既存セクション番号を§11〜§13へ繰り下げ
- config.example.toml: [prompt]コメント例追記
- doc/Specifications.md: モデル構成表にBase prompt行追加、推奨ディレクトリに
  prompts/追記、設定例更新
- doc/Design.md: 設定オブジェクトにPromptConfig追記、モデルパス解決表に
  prompts/追加
- doc/mcp.md: §2.1にbase prompt結合の記載追記
- README.md: [prompt]設定例と結合規則の説明追記

検証:
- ruff format/check、pyright 0 errors、pytest 61 passed
- 実GPU・実モデルでstreamable-http経由generate_imageでbase prompt結合を確認
  positive='masterpiece, best quality, ..., sharp focus, a small red cube on a white table'
  negative='low quality, ..., jpeg artifacts\nnoise'
  base前置・user後置の結合順序が仕様通り機能することをサーバログで検証済み

3 files changed, 138 insertions, 0 deletions

diff --git a/tests/conftest.py b/tests/conftest.py
index 862b4f5..e4a7c4b 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -14,6 +14,7 @@ from mcp_sd15.config import (
     GenerationConfig,
     ModelConfig,
     OutputConfig,
+    PromptConfig,
     ServerConfig,
 )
 
@@ -30,6 +31,7 @@ def app_config(tmp_path: Path) -> AppConfig:
         ),
         output=OutputConfig(tmp_path / "outputs", False),
         server=ServerConfig(),
+        prompt=PromptConfig(),
     )
 
 
diff --git a/tests/test_config.py b/tests/test_config.py
index 9a6bf89..b0dfac0 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -195,3 +195,77 @@ def test_rejects_empty_host(tmp_path: Path) -> None:
     )
     with pytest.raises(ConfigError, match="host"):
         load_config(path, validate_paths=False)
+
+
+def test_prompt_defaults_to_empty_when_omitted(tmp_path: Path) -> None:
+    path = tmp_path / "config.toml"
+    path.write_text(
+        f'[model]\nroot = "{tmp_path.as_posix()}"\ncheckpoint = "base.safetensors"\n',
+        encoding="utf-8",
+    )
+    config = load_config(path, validate_paths=False)
+    assert config.prompt.positive == ""
+    assert config.prompt.negative == ""
+
+
+def test_prompt_reads_files_from_root_prompts(tmp_path: Path) -> None:
+    root = tmp_path / "models"
+    (root / "checkpoints").mkdir(parents=True)
+    (root / "checkpoints" / "base.safetensors").touch()
+    (root / "prompts").mkdir()
+    (root / "prompts" / "positive.txt").write_text("masterpiece, best quality", encoding="utf-8")
+    (root / "prompts" / "negative.txt").write_text("low quality, blurry", encoding="utf-8")
+    path = tmp_path / "config.toml"
+    path.write_text(
+        f'[model]\nroot = "{root.as_posix()}"\ncheckpoint = "base.safetensors"\n'
+        '[prompt]\npositive_file = "positive.txt"\nnegative_file = "negative.txt"\n',
+        encoding="utf-8",
+    )
+    config = load_config(path)
+    assert config.prompt.positive == "masterpiece, best quality"
+    assert config.prompt.negative == "low quality, blurry"
+
+
+def test_prompt_rejects_path_traversal(tmp_path: Path) -> None:
+    root = tmp_path / "models"
+    (root / "checkpoints").mkdir(parents=True)
+    (root / "checkpoints" / "base.safetensors").touch()
+    path = tmp_path / "config.toml"
+    path.write_text(
+        f'[model]\nroot = "{root.as_posix()}"\ncheckpoint = "base.safetensors"\n'
+        '[prompt]\npositive_file = "../escape.txt"\n',
+        encoding="utf-8",
+    )
+    with pytest.raises(ConfigError, match="file name"):
+        load_config(path, validate_paths=False)
+
+
+def test_prompt_rejects_missing_file(tmp_path: Path) -> None:
+    root = tmp_path / "models"
+    (root / "checkpoints").mkdir(parents=True)
+    (root / "checkpoints" / "base.safetensors").touch()
+    path = tmp_path / "config.toml"
+    path.write_text(
+        f'[model]\nroot = "{root.as_posix()}"\ncheckpoint = "base.safetensors"\n'
+        '[prompt]\npositive_file = "absent.txt"\n',
+        encoding="utf-8",
+    )
+    with pytest.raises(ConfigError, match="prompt file"):
+        load_config(path, validate_paths=False)
+
+
+def test_prompt_accepts_only_positive(tmp_path: Path) -> None:
+    root = tmp_path / "models"
+    (root / "checkpoints").mkdir(parents=True)
+    (root / "checkpoints" / "base.safetensors").touch()
+    (root / "prompts").mkdir()
+    (root / "prompts" / "positive.txt").write_text("masterpiece", encoding="utf-8")
+    path = tmp_path / "config.toml"
+    path.write_text(
+        f'[model]\nroot = "{root.as_posix()}"\ncheckpoint = "base.safetensors"\n'
+        '[prompt]\npositive_file = "positive.txt"\n',
+        encoding="utf-8",
+    )
+    config = load_config(path)
+    assert config.prompt.positive == "masterpiece"
+    assert config.prompt.negative == ""
diff --git a/tests/test_schema.py b/tests/test_schema.py
index 15a5f9b..51ec4a2 100644
--- a/tests/test_schema.py
+++ b/tests/test_schema.py
@@ -41,3 +41,65 @@ def test_inpaint_requires_both_images() -> None:
 def test_arbitrary_dimensions_are_not_accepted() -> None:
     with pytest.raises(InvalidRequestError, match="size"):
         TextToImageRequest.create("cat", size="640x480")
+
+
+def test_base_prompts_are_combined_with_user_prompts() -> None:
+    request = TextToImageRequest.create(
+        "a cat",
+        "blurry",
+        base_positive="masterpiece, best quality",
+        base_negative="low quality, watermark",
+    )
+    assert request.positive_prompt == "masterpiece, best quality, a cat"
+    assert request.negative_prompt == "low quality, watermark\nblurry"
+
+
+def test_base_positive_only_does_not_add_trailing_separator() -> None:
+    request = TextToImageRequest.create(
+        "a cat",
+        base_positive="masterpiece",
+    )
+    assert request.positive_prompt == "masterpiece, a cat"
+    assert request.negative_prompt == ""
+
+
+def test_base_negative_only_does_not_add_leading_separator() -> None:
+    request = TextToImageRequest.create(
+        "a cat",
+        "blurry",
+        base_negative="low quality",
+    )
+    assert request.positive_prompt == "a cat"
+    assert request.negative_prompt == "low quality\nblurry"
+
+
+def test_empty_base_prompts_preserve_user_prompts() -> None:
+    request = TextToImageRequest.create("a cat", "blurry")
+    assert request.positive_prompt == "a cat"
+    assert request.negative_prompt == "blurry"
+
+
+def test_guided_request_combines_base_prompts() -> None:
+    request = GuidedImageRequest.create(
+        "a dancer",
+        "openpose",
+        "abc",
+        "deformed",
+        base_positive="masterpiece",
+        base_negative="low quality",
+    )
+    assert request.positive_prompt == "masterpiece, a dancer"
+    assert request.negative_prompt == "low quality\ndeformed"
+
+
+def test_inpaint_request_combines_base_prompts() -> None:
+    request = InpaintImageRequest.create(
+        "a vase",
+        "abc",
+        "def",
+        "artifacts",
+        base_positive="masterpiece",
+        base_negative="low quality",
+    )
+    assert request.positive_prompt == "masterpiece, a vase"
+    assert request.negative_prompt == "low quality\nartifacts"