Resume existing task instead of sending new input

Currently, when the create-task action finds an existing task with the same name, it waits for the task to become active/idle and then sends the prompt as new input via `sendTaskInput`. This does not resume the original task's work — it gives the task a brand new instruction.

The action should attempt to resume the existing task (i.e. continue/retry the original work) rather than just sending new input to it.

## Current Behavior

1. `getTask()` finds an existing task by name
2. `waitForTaskActive()` waits for the task to be active and idle
3. `sendTaskInput()` sends the prompt as new input to the existing task
4. Returns `taskCreated: false`

## Expected Behavior

When an existing task is found, the action should attempt to resume/retry the task's original work.

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Created on behalf of @johnstcn