feat(phase3): add The Librarian agent with library-desk integration

Library-Desk API Client:
- Async HTTP client with httpx for library-desk API
- HybridRAG search (vector + graph + web)
- Wiki operations (search, get, list, create, update)
- Smart page creation with HybridRAG research
- Semantic vector search and knowledge graph queries
- Dossier browsing and health checks

Librarian Tools (11 total):
- Research: hybrid_search, search_wiki, get_wiki_page, semantic_search
- Browse: list_dossiers, get_dossier_pages, explore_knowledge_graph
- Graph: find_related_entities
- Write: create_wiki_page, update_wiki_page, smart_create_wiki_page

Agent:
- PydanticAI agent with research assistant personality
- System prompt with research and writing workflows
- Streaming support via run_librarian_stream()

Capability:
- LIBRARIAN_CAPABILITY definition for Household Registry
- Automatic registration on startup

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agents/librarian/capability.py

@@ -0,0 +1,81 @@
+"""
+Librarian capability registration for the Household Registry.
+
+Defines The Librarian's capabilities and registers it as a
+household member for coordination by the Steward and Tatlock.
+"""
+from src.agents.librarian.agent import get_librarian_agent
+from src.agents.librarian.tools import LIBRARIAN_TOOLS
+from src.core.household_registry import (
+    HouseholdCapability,
+    get_household_registry,
+)
+from src.core.logging_config import get_logger
+
+logger = get_logger(__name__)
+
+
+# The Librarian's capability summary for Steward coordination
+LIBRARIAN_CAPABILITY = HouseholdCapability(
+    name="librarian",
+    role="The Librarian",
+    category="research",
+    description=(
+        "Research assistant providing knowledge search, wiki access, "
+        "semantic search, and knowledge graph exploration via library-desk API"
+    ),
+    domains=[
+        "research",
+        "knowledge",
+        "information",
+        "wiki",
+        "documents",
+        "search",
+        "synthesis",
+    ],
+    cost="medium",  # Multiple API calls to library-desk
+    requires_network=True,  # Needs library-desk API access
+)
+
+
+def get_librarian_capability() -> HouseholdCapability:
+    """Get The Librarian's capability definition."""
+    return LIBRARIAN_CAPABILITY
+
+
+def register_librarian() -> None:
+    """
+    Register The Librarian with the Household Registry.
+
+    This makes The Librarian available for:
+    - Steward recommendations (via capability summary)
+    - Tatlock delegation (via agent reference)
+    - Tool scoping (via tool list)
+    """
+    registry = get_household_registry()
+
+    # Check if already registered
+    if "librarian" in registry:
+        logger.debug("librarian_already_registered")
+        return
+
+    registry.register(
+        name="librarian",
+        capability=LIBRARIAN_CAPABILITY,
+        tools=LIBRARIAN_TOOLS,
+        agent=get_librarian_agent(),
+    )
+
+    logger.info(
+        "librarian_registered",
+        role=LIBRARIAN_CAPABILITY.role,
+        domains=LIBRARIAN_CAPABILITY.domains,
+        tool_count=len(LIBRARIAN_TOOLS),
+    )
+
+
+def unregister_librarian() -> None:
+    """Unregister The Librarian from the Household Registry."""
+    registry = get_household_registry()
+    registry.unregister("librarian")
+    logger.info("librarian_unregistered")