PULL_REQUEST_TEMPLATE.md

## Which issue does this PR address?

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Closes #
Relates to #

## Rationale

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Why is this change needed? If the issue explains it well, a one-liner is fine.
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## What changed?

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2-4 sentences. Problem first (before), then solution (after).

GOOD:

"Messages were unavailable when background message_saver committed the
journal and started async disk I/O before completion. Polling during
this window found neither journal nor disk data.

The fix freezes journal batches in the in-flight buffer before async persist."

GOOD:

"When many small messages accumulate in the journal, the flush passes
thousands of IO vectors to writev(), exceeding IOV_MAX (1024 on Linux)."

BAD:
- Walls of text
- "This PR adds..." (we can see the diff)
-->

## Local Execution

- Passed / not passed
- Pre-commit hooks ran / not ran

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## AI Usage

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