Add task timeout option to prevent long-running tasks

[Copilot]

This PR implements a task timeout feature that allows users to specify a maximum execution time for tasks. If a task runs longer than the specified timeout, it will be automatically terminated with a clear error message.

Changes Made

Schema Updates:

• Added optional timeout property to all task schemas (run.yml, exec.yml, signal.yml)
• Timeout value must be a positive integer representing seconds
• Property is optional to maintain backward compatibility

Implementation:

• RunTask: Uses Promise.race() to compete docker run execution against timeout, with container cleanup on timeout
• ExecTask: Implements timeout for docker exec operations with proper stream handling
• SignalTask: Adds timeout support for signal sending operations
• All task types properly clean up timeout handlers to prevent memory leaks

Documentation:

• Updated docs/tasks.md with timeout parameter description and examples
• Added usage examples showing timeout: 30 syntax

Testing:

• Added comprehensive tests for all three task types covering both success and timeout scenarios
• Tests verify proper error messages and cleanup behavior
• All existing tests continue to pass (35/35)

Usage Example

tasks:
  backup-data:
    type: run
    image: busybox
    command: ./backup-script.sh
    timeout: 300  # 5 minute timeout
  
  health-check:
    type: exec
    container: web-server
    command: curl -f http://localhost/health
    timeout: 10   # 10 second timeout

Tasks that exceed their timeout will fail with a clear error message: Task timed out after X seconds

This feature helps prevent jobs from hanging indefinitely due to stuck containers or long-running operations, improving the reliability and predictability of scheduled tasks.

Fixes #12.

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