fix: write transcript completion marker last

[sumajoo]

Summary

• write transcript.md before transcript.json
• keep transcript.json as the final completion marker for a session
• add a minimal Swift Testing target with a regression test

Why

resumePending() treats the presence of transcript.json as “done”. Previously,
the JSON file was written before the Markdown transcript. If the Markdown write
failed, the JSON marker remained and the session was never retried.

Writing the Markdown artifact first and the JSON marker last preserves the
existing successful output while leaving failed sessions pending for retry.

Validation

• swift test
• swift build
• swift build -c release
• git diff --check

The regression test fails with the previous write order and passes with this
change.

[sollrr9]

🤖 Grok (xAI) PR Review — model: grok-4.5

verdict: LGTM
tests: not run: swift unavailable on review host (macOS package)
linked_issues: none

Issues (must-fix)

• none

Suggestions

• windows/TranscriptionCoordinator.cs:326 — same JSON-before-MD order; mirror marker-last there
• Tests/quillTests/Transcription/TranscriptTests.swift:27 — add happy-path assert both files exist after write

Next steps: merge when green | optional Windows parity follow-up