feat: add skills system for specialized agent workflows (#6)

Implement a skills framework that enables specialized workflows for
specific tasks (e.g., PDF processing, web page generation). Skills are
discovered from the skills/ directory and automatically mounted in
sandboxes with path mapping support.

- Add SkillsConfig for configuring skills path and container mount point
- Implement dynamic skill loading from SKILL.md files with YAML frontmatter
- Add path mapping in LocalSandbox to translate container paths to local paths
- Mount skills directory in AIO Docker sandbox containers
- Update lead agent prompt to dynamically inject available skills
- Add setup documentation and expand config.example.yaml

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/config/app_config.py

@@ -8,6 +8,7 @@ from pydantic import BaseModel, ConfigDict, Field
 
 from src.config.model_config import ModelConfig
 from src.config.sandbox_config import SandboxConfig
+from src.config.skills_config import SkillsConfig
 from src.config.title_config import load_title_config_from_dict
 from src.config.tool_config import ToolConfig, ToolGroupConfig
 
@@ -21,6 +22,7 @@ class AppConfig(BaseModel):
     sandbox: SandboxConfig = Field(description="Sandbox configuration")
     tools: list[ToolConfig] = Field(default_factory=list, description="Available tools")
     tool_groups: list[ToolGroupConfig] = Field(default_factory=list, description="Available tool groups")
+    skills: SkillsConfig = Field(default_factory=SkillsConfig, description="Skills configuration")
     model_config = ConfigDict(extra="allow", frozen=False)
 
     @classmethod