feat: per-task description field for bot context

[jlunder00]

What

Tasks currently have only a text field (short title). Add an optional description field per task — visible to the bot for context but hidden by default in the UI.

Why

Keeps task titles short and scannable while giving the bot enough context to understand what the task involves without stuffing details into the title.

Changes needed

DB

• Add description TEXT NOT NULL DEFAULT '' column to tasks table (migration needed)

db/queries.py

• upsert_tasks accepts list[dict] with text + optional description instead of list[str]
• get_plan returns description per task

Mutation format

• update_plan_tasks tasks become objects: [{"text": "Message Jacob Kranz", "description": "FAIR connection via GU/De Palma — do not apply cold"}]
• Backwards compat: plain strings still accepted (description defaults to "")

MCP

• update_plan_tasks tool updated to accept task objects

Frontend

• Tasks render as short titles (unchanged)
• Optional "show detail" toggle per task to reveal description
• Bot-written descriptions not shown by default

Prompt

• Bot instructed: task text = short title, description = context/detail (optional, omit if not needed)