fix(tool-cache): persist runner diagnostic events

[rldyourmnd]

Summary

• keep the existing structured job-console event
• mirror the same bounded non-secret record into the runner _diag directory
• fail open when a nonstandard runner does not expose a safe writable diagnostic directory
• test byte-for-byte parity between console and diagnostic evidence

Root cause

Natural run NDDev-it-com/captcha#32783048547 emitted four tool-cache markers in the GitHub job log, but its verified teardown bundle for runner nddev-w2rnoibatea3 contained zero markers across four runner diagnostic files. GitHub Runner does not persist action stdout in _diag.

Verification

• bash -n actions/tool-cache/tool-cache.sh
• go test -race ./internal/repositorycontract ./internal/diagnosticexport
• make verify