bonk: improve PR review steering

[elithrar]

Improve prompt to steer PR review comments.

• Force the $PR_NUMBER into the prompt
• Improved fetch-depth for diffs (balance of larger checkouts / time)
• Stronger prompt guidance

These are also now exposed in Bonk's GitHub Action (which feeds into OpenCode) as of ask-bonk/ask-bonk#117

[dom96]

Hmm, I guess the fact that it did it again in this PR suggests that it's a bug in Bonk rather than a prompt issue.

[codspeed-hq]

Merging this PR will not alter performance

✅ 70 untouched benchmarks
⏩ 129 skipped benchmarks¹

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_{Comparing elithrar-patch-1 (b25819b) with main (1a5413f)}

Footnotes

1. 129 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩