Extend AI agent with due-date filtering #7

Fast-follow on due dates — extend the agent's task-listing tool with date-range
filtering so it can answer "what's due this week", "show overdue tasks", etc.

What shipped

• Extended list_tasks_tool with a due_within_days parameter rather than adding
  a new tool
• Found and fixed a tool-selection bug: the agent was routing date-related queries
  to the RAG search tool instead, which has no due-date data
• Found and fixed a second bug: "overdue" phrasing wasn't mapping to the correct
  filter argument, causing the agent to list all tasks unfiltered
• Surfaced a separate infra bug during debugging — filed as LED-160: Fix Chroma persist directory path ambiguity #4

PR: #3