docs(AGENTS): prefer Rrbvec over O(n²) List concat patterns

[RCmerci]

Summary

• Creates AGENTS.md with OCaml guidance: avoid O(n²) List patterns such as List.concat and repeated List.append on large sequences; when the project already depends on the rrbvec package, use Rrbvec vectors instead.

Link to Devin session: https://app.devin.ai/sessions/cc13a6a77cdd40f89ea6695bff8ff7eb
Open in Devin Desktop: https://app.devin.ai/desktop/session/cc13a6a77cdd40f89ea6695bff8ff7eb?variant=devin
Requested by: @RCmerci

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

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[chatgpt-codex-connector]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-27T01:12:39.647165Z	f05699d	PR opened

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[chatgpt-codex-connector]

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Reviewed commit: f05699d330

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[chatgpt-codex-connector]

Do not classify every List.concat as quadratic

When an agent touches existing flattening code such as lib/syntax/inline.ml:952, this rule incorrectly labels it an O(n²) violation: OCaml's List.concat is linear in the total number of list elements. This can prompt needless Rrbvec rewrites or dependency changes; narrow the guidance to repeated appends or other incremental concatenation patterns that repeatedly rescan prefixes.

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