feat(wellness): add update_wellness write tool, computed Form (TSB), date-label fix

Add an update_wellness MCP tool that writes nutrition macros, hydration, vitals,
sleep, and subjective ratings to Intervals.icu via PUT /athlete/{id}/wellness/{date}.
Only provided fields are sent; pass -1 to clear a numeric field and locked=True to
stop device/app syncs from overwriting the values. Sleep is taken in hours and
stored as seconds (with -1 passing through as the clear sentinel).

Also surface computed Form (TSB = CTL - ATL) in wellness output, and prefer an
explicit `date` field over the record `id` for the Date label.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NGzHtDvJur9U7ysgRKRUTN

src/intervals_mcp_server/tools/__init__.py

@@ -32,7 +32,10 @@ from intervals_mcp_server.tools.power_curves import (  # noqa: F401
     get_athlete_power_curves,
 )
 from intervals_mcp_server.tools.gear import get_gear_list  # noqa: F401
-from intervals_mcp_server.tools.wellness import get_wellness_data  # noqa: F401
+from intervals_mcp_server.tools.wellness import (  # noqa: F401
+    get_wellness_data,
+    update_wellness,
+)
 
 
 def register_tools(mcp_instance: FastMCP) -> None:
@@ -70,4 +73,5 @@ __all__ = [
     "get_athlete_power_curves",
     "get_gear_list",
     "get_wellness_data",
+    "update_wellness",
 ]