fix: don't report user input errors to the Blacksmith backend

[piob-io]

The v2 API deliberately made cache-key a required input. Workflows still using the v1 invocation style hit the @actions/core required-input throw (Input required and not supplied: cache-key) inside getStickyDisk, which the setup error path then shipped to /stickydisks/report-failed as a STICKYDISK_SETUP failure. That pollutes platform failure metrics with pure user configuration errors and has been tripping the per-region failure-rate paging alert (~6k reports/day fleet-wide). useblacksmith/web#10236 currently filters this backend-side by string-matching the message; this PR fixes it at the source so no string matching is needed anywhere.

User misconfiguration is now modeled as a distinct error type instead of an error string. getStickyDisk validates cache-key itself (up front, before creating the agent client) and throws UserInputError with an actionable v2 migration message, and the reporter structurally refuses to send any UserInputError to the backend:

export async function reportBuildPushActionFailure(type, error?, event?) {
  if (error instanceof UserInputError) {
    core.debug(`Not reporting user input error to Blacksmith: ${error.message}`);
    return;
  }
  // ... report as before
}

Guarding centrally in reportBuildPushActionFailure (rather than at the setupStickyDisk call site) means every present and future call site is covered.

User-facing behavior is unchanged: the error still propagates exactly as before, so the action warns and falls back to the local builder (or fails the job when nofallback is set), just with a clearer message telling the user to add cache-key.

dist/index.js and dist/index.js.map are updated with the CI-built bundle for this source, matching repo convention.

Summary

• New UserInputError type for workflow misconfiguration errors
• getStickyDisk validates cache-key itself and throws UserInputError with a v2 migration hint
• reportBuildPushActionFailure never reports UserInputError to the backend
• Unit tests for both the validation and the reporting guard

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