docs: document due dependencies in README

[Copilot]

PR #49 introduced task dependency gating, but README did not describe how dependency state affects task urgency or how to configure dependencies. This update adds focused documentation so users can discover and correctly use due dependencies from the main docs.

• Feature surface update

  • Added Task Dependencies to the top-level Features list.

• Configuration docs

  • Added Dependencies to the task Available Options table.
  • Clarified that dependencies are selected from other Task Tracker sensor entities.

• Behavior and constraints

  • Added a dedicated Due dependencies section describing:
    • urgency clamping model (done < due_soon < due)
    • immediate re-evaluation when dependency state changes
    • circular dependency prevention
    • validation that only Task Tracker sensors are allowed

• Example (as documented)

  #### Due dependencies
  
  Use **Dependencies** to make a task wait on other Task Tracker tasks:
  
  - A task cannot become more urgent than its least urgent dependency (`done` < `due_soon` < `due`)
  - If a dependency changes state, dependent tasks update immediately
  - Circular dependencies are blocked
  - Only Task Tracker sensors can be selected as dependencies