feat(mcp): event CRUD tools

Five new tools (events weren't in fable-mcp before). Split
start_date + start_time inputs combine into a naive datetime that
services/events.py interprets in the user's local timezone.

Sentinels for update:
  - empty strings → leave unchanged
  - duration_minutes=-1 → leave unchanged
  - duration_minutes=0 → set to point event (NULL duration)
  - start_date/start_time must BOTH be set to move the event

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/fabledassistant/mcp/tools/__init__.py

@@ -5,7 +5,7 @@ to a FastMCP instance. `register_all(mcp)` is the single entry point called
 from `mcp.server.build_mcp_server`.
 """
 from fabledassistant.mcp.tools import (
-    milestones, notes, projects, search, tasks,
+    events, milestones, notes, projects, search, tasks,
 )
 
 
@@ -16,3 +16,4 @@ def register_all(mcp) -> None:
     tasks.register(mcp)
     projects.register(mcp)
     milestones.register(mcp)
+    events.register(mcp)