feat: expose compiled token data as job outputs or artifact (not just step summary)

[adamhenson]

Context

PR #23943 is a great step — the compiler now emits a token usage step that renders
token-usage.jsonl to $GITHUB_STEP_SUMMARY. The data is there and visible to humans.

Gap

The token data isn't accessible to downstream jobs or third-party tools at workflow
runtime. The step summary is write-only from a workflow perspective — no downstream
job can read ${{ needs.agent.outputs.input_tokens }} or download a structured artifact.

Proposal

In the compiled token step, also:

1. Write to $GITHUB_OUTPUT so the agent job exposes outputs:

   outputs:
     input_tokens: ${{ steps.token_usage.outputs.input_tokens }}
     output_tokens: ${{ steps.token_usage.outputs.output_tokens }}
     cache_read_tokens: ${{ steps.token_usage.outputs.cache_read_tokens }}
     cache_write_tokens: ${{ steps.token_usage.outputs.cache_write_tokens }}

[adamhenson]

😍