Document docs spell check pre-push gate in AGENTS.md

[bdraco]

What do these changes do?

Adds a new PR rule to AGENTS.md that documents make doc-spelling as the pre-push gate, mirroring aio-libs/multidict#1345. The new section explains that sphinxcontrib.spelling reads every CHANGES/*.rst fragment, so an unknown technical word in a news fragment fails CI before a human ever sees the PR; the fix is to either add the word to docs/spelling_wordlist.txt in the same commit or correct the typo.

Renumbers the surrounding PR rules (Commit hygiene becomes #7) and adds a matching bullet to Things not to do.

Are there changes in behavior for the user?

No. Docs-only change to the agent-facing contributor notes.

Is it a substantial burden for the maintainers to support this?

No. make doc-spelling is already a CI gate via reusable-linters.yml; this just writes the rule down where agents will read it.

Related issue number

Mirrors aio-libs/multidict#1345.

Checklist

• I think the code is well written
• [N/A] Unit tests for the changes exist
• Documentation reflects the changes
• [N/A] If you provide code modification, please add yourself to CONTRIBUTORS.txt
• Add a new news fragment into the CHANGES/ folder

[codspeed-hq]

Merging this PR will not alter performance

✅ 99 untouched benchmarks

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_{Comparing bdraco:agents-doc-spelling (3356871) with master (faaa1fd)¹}

Footnotes

1. No successful run was found on master (255818d) during the generation of this report, so faaa1fd was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩