Add Discord attachment reading and web search capabilities

- Discord channel now downloads and extracts text from attachments (text files, PDFs)
- Added WebSearchTool using DuckDuckGo for researcher and coder agents
- Improved WebFetchTool with User-Agent header and HTML-to-text stripping
- Added pypdf and duckduckgo-search dependencies

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

xtrm_agent/bus.py

@@ -8,6 +8,14 @@ from datetime import datetime, timezone
 from typing import Any
 
 
+@dataclass
+class Attachment:
+    """A text-extracted attachment from a user message."""
+
+    filename: str
+    content: str
+
+
 @dataclass
 class InboundMessage:
     """Message from a channel (user) heading to an agent."""
@@ -19,6 +27,7 @@ class InboundMessage:
     target_agent: str | None = None
     timestamp: datetime = field(default_factory=lambda: datetime.now(timezone.utc))
     metadata: dict[str, Any] = field(default_factory=dict)
+    attachments: list[Attachment] = field(default_factory=list)
 
 
 @dataclass