feat: expose token usage as step outputs

[adamhenson]

Closes #59, closes #136.

The action tracks token usage internally but never surfaces it as step outputs, so workflows can't gate on cost, log spend, or forward counts to external tools. This PR wires it up.

Changes

base-action/src/run-claude-sdk.ts — Extended ClaudeRunResult with 5 new optional fields. Added accumulators in the SDK message loop summing message.usage from each assistant message, and set them on the result after the loop alongside num_turns from resultMessage.

src/entrypoints/run.ts — 5 new core.setOutput() calls after conclusion, using ?? "" as fallback (same pattern as the rest of the codebase).

action.yml — 5 new entries in the outputs: section.

docs/usage.md — New ## Tracking Token Usage and Cost section inserted between Structured Outputs and Ways to Tag @claude, with sub-sections covering logging, budget enforcement, and forwarding to a cost dashboard.

Usage after this PR

- uses: anthropics/claude-code-action@v1
  id: claude
  with:
    anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
    prompt: "..."

- name: Log token usage
  run: |
    echo "Input tokens:  ${{ steps.claude.outputs.input_tokens }}"
    echo "Output tokens: ${{ steps.claude.outputs.output_tokens }}"
    echo "Turns:         ${{ steps.claude.outputs.turns }}"

Budget enforcement is now possible via a simple workflow step checking the outputs — partially addressing #136.