feat(ecs): add AWS task tag support

[Will Graham (wlggraham)]

Summary

• Adds a tags field to the ECS cluster context, command context, and job context so operators can attach arbitrary AWS resource tags to tasks at launch
• Tags are merged in precedence order: cluster < command < job, so more specific contexts can override broader defaults
• job_id is always injected automatically so every ECS task is traceable back to its Heimdall job

How it works

Tags from all three levels are resolved once in buildExecutionContext before any tasks are launched:

1. Cluster tags form the base
2. Command tags overlay cluster tags (json.Unmarshal merges map fields, so command and job tags are already merged by the time they reach the execution context)
3. job_id is set last and cannot be overridden

Example config

# Cluster context
tags:
  env: prod
  cluster: ecs-fargate

# Command context
tags:
  team: data-platform

# Job context (at submission time)
tags:
  pipeline: my-pipeline

Resulting ECS task tags: env=prod, cluster=ecs-fargate, team=data-platform, pipeline=my-pipeline, job_id=<heimdall-job-id>

Test plan

• Submit an ECS job and verify tags appear on the task in the AWS console
• Confirm job_id tag is always present
• Confirm job-level tags override cluster/command tags on the same key
• Confirm jobs with no tags configured at any level still launch without error