fix: expose conclusion as a root action output

[fallintoplace]

Summary

The unified entrypoint sets core.setOutput("conclusion", claudeResult.conclusion), but the root action.yml does not declare or forward that output. Callers therefore cannot reliably consume steps.<id>.outputs.conclusion, even though the internal run step produces it.

This adds the missing public output mapping:

conclusion:
  description: "Execution status of Claude Code ('success' or 'failure')"
  value: ${{ steps.run.outputs.conclusion }}

base-action/action.yml already exposes the same output, so this also makes the root and standalone action interfaces consistent.

Regression coverage

Adds a focused metadata test that checks the root output forwards steps.run.outputs.conclusion. This catches future drift between outputs set by the entrypoint and outputs exposed by the composite action.

Verification

• bun test: 805 pass, 0 fail
• bun run typecheck: clean
• bunx prettier --check action.yml test/action-metadata.test.ts: clean
• git diff --check: clean