docs(mcp): instruct agents to drive task lifecycle + log work

Adds a 'keep task state honest' directive to the MCP _INSTRUCTIONS: set
in_progress on start, log progress with add_task_log as you go, set done the
moment work completes (never leave finished work at todo), and write a dated
dev-log note on the project at significant landings. Reinforced in the
update_task status docstring. App-layer + always-loaded, no rule/config needed
— closes the gap where finished work (e.g. a shipped plan) sat open because the
lifecycle was available but never prescribed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/fabledassistant/mcp/tools/tasks.py

@@ -143,7 +143,9 @@ async def update_task(
         task_id: ID of the task to update.
         title: New title, or omit to leave unchanged.
         body: New markdown body, or omit to leave unchanged.
-        status: New status — one of: todo, in_progress, done, cancelled.
+        status: New status — one of: todo, in_progress, done, cancelled. Drive
+            the lifecycle: set in_progress when you start, done when complete —
+            don't leave finished work at todo.
         priority: New priority — one of: none, low, medium, high.
         project_id: New project. Omit (0) to leave unchanged.
         milestone_id: New milestone. Omit (0) to leave unchanged.