#4472 fix(journal): clarify failure diagnostics

Summary

Keep application error codes out of the journal diagnostic's filesystem errno field while preserving normalized codes and typed reasons. Document how to capture missing-event diagnostics with --debug, where request and trace records are stored, and that capacity includes the active write.

Four files changed. Follow-up to three accepted late review comments on #4470, related to #4443 and umbrella #4368. No scope expansion or change to the adopted temporary-file policy.

Validation

Head 3cabc4af3b0500e6fbb51921057dd3cf49ea3982; base d3a55ae6e6b7412c74464bc28c87c7185d55f2eb.

• Pinned pnpm 11.17.0 frozen install/build and one check:affected --run --head 3cabc4af3b0500e6fbb51921057dd3cf49ea3982 --json passed. Receipt: exactCommit=true, checkoutChanged=false, failOpen=false; clean before/after. Resolved main 59c52465a7ef95323ab859898f7a562c868e392f; merge base is the base above.
• All eleven runnable stages passed: 342 related tests, full unit 19,432 passed/3 skipped, command docs 12 passed, plus format/lint/typecheck/layering/Fallow/build/docs links/prose. Seven GitHub-owned or parked manual lanes were skipped.
• Identical focused test command with the added assertions: base production 20 passed/1 intended failure (COMMAND_FAILED in errno); corrected production 21 passed, including filesystem EACCES coverage.
• Independent source review found no concrete regressions. No native behavior changed; no device run required. Current-head CI has 21 successes and one skipped cache-build lane. Sentinel approves with no findings; Cubic reports no issues. GitHub reports MERGEABLE/CLEAN; the formal review decision is empty. #4050 and test budgets are unchanged.