Report agent assignment errors in workflow failure tracking

[Copilot]

Agent assignment failures (due to missing or invalid GH_AW_AGENT_TOKEN) were silently failing without being reported in the failure tracking issue. This made it difficult to diagnose why assignments weren't happening.

Changes

• Capture assignment errors: assign_to_agent.cjs now exports assignment_errors and assignment_error_count outputs containing failure details
• Expose through workflow: Compiler adds these outputs to the safe_outputs job in generated workflows
• Pass to conclusion job: notify_comment.go supplies assignment error data as environment variables
• Report in failure issues: handle_agent_failure.cjs processes assignment errors even when agent job succeeds, formatting them in failure issues/comments

Result

Failure issues now display assignment errors with issue numbers and error messages:

⚠️ Agent Assignment Failed: Failed to assign agent to issues due to insufficient permissions or missing token.

Assignment Errors:
- Issue #11198 (agent: copilot): Failed to assign copilot via GraphQL
- Issue #11136 (agent: copilot): Insufficient permissions

Error format: {type}:{number}:{agent}:{error_message} where type is "issue" or "pr".

Original prompt

Reference: https://github.com/githubnext/gh-aw/actions/runs/21238712945/job/61112047802#step:7:1

When assigning the agent fails (because of missing token or invalid token), report the error as an output variable.

In the conclusion job, review this output variable and add it to the "failed runs" issue or comment created when a problem is detected.

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