feat(agents): p50-lateness tolerance + per-project realness for overdue-task (#115)

Replaces snooze-rate heuristic with p50 of actual task lateness (completedAt − dueAt).
Adds project_realness inference: projects with chronic lateness get realness < 1 and
the agent softens its snippet language from "overdue" to "past target date".

- TaskCompletion added to UserHistory with lateness_days computed property
- _infer_lateness_tolerance: p50 of task_completions, clipped at 0, float
- _infer_project_realness: per-project median lateness normalised by global median
- Both InferredParams use 7d TTL; cold_start = 0.0 / {}
- AgentInferRequest accepts task_completions; endpoint wires them through
- 12 new tests covering punctual/chronic/mixed users and language softening
- Agent bumped to v1.2.0

Closes #115

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

ml/agents/inference/__init__.py

@@ -4,6 +4,6 @@ Each agent's manifest declares InferredParams; this package owns the
 scheduling contract, history data model, and write path to user_preferences.
 """
 from .framework import run_inference
-from .history import FeedbackEvent, UserHistory
+from .history import FeedbackEvent, TaskCompletion, UserHistory
 
-__all__ = ["run_inference", "FeedbackEvent", "UserHistory"]
+__all__ = ["run_inference", "FeedbackEvent", "TaskCompletion", "UserHistory"]