Add manual test doc for .quarto_ipynb cleanup on ungraceful termination

Author: cderv

tests/docs/manual/preview/14359-positron-kill-quarto-ipynb.md

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+# Manual Test: .quarto_ipynb cleanup on ungraceful termination (#14359)
+
+## Purpose
+
+Verify that `.quarto_ipynb` files are cleaned up after render even when preview
+is killed ungracefully (e.g., Positron terminal bin icon). This tests the fix
+that restores immediate deletion in `cleanupNotebook()`.
+
+**Why manual?** Ungraceful termination bypasses normal cleanup handlers.
+The fix ensures deletion happens right after execution, before any signal
+could interrupt the process.
+
+## Automated coverage
+
+The smoke-all test `tests/docs/smoke-all/2025/05/21/keep_ipynb_single-file/14359.qmd`
+verifies the core behavior (file deleted after render). This manual test
+covers the interactive preview scenario.
+
+## Steps
+
+1. Open Positron (or VS Code with Quarto extension)
+2. Open `tests/docs/manual/preview/14281-quarto-ipynb-accumulation.qmd`
+3. Start Quarto Preview (terminal or Quarto extension)
+4. Wait for first render to complete
+5. Check directory: `ls *.quarto_ipynb*` — should be zero files (file was deleted after render)
+6. Edit and save the file to trigger a re-render
+7. After re-render completes, check again — still zero `.quarto_ipynb` files
+8. Kill preview ungracefully (Positron terminal bin icon / close terminal)
+9. Check directory — still zero `.quarto_ipynb` files
+
+## Expected
+
+- Zero `.quarto_ipynb` files at all times (the file is deleted immediately
+  after each execution, not deferred to cleanup handlers)
+- Ungraceful termination does not leave stale files because deletion already
+  happened
+
+## Related
+
+- #14281 — within-session accumulation (fixed by PR #14287)
+- #12780 — `keep-ipynb` support (PR #12793 introduced this regression)
+- posit-dev/positron#13006 — Killing Quarto Preview should exit process