Add detailed AGENTS.md files for most of the modules

[zaleslaw]

Added comprehensive CLAUDE.md files for the core library and all modules (dataframe-json, dataframe-arrow, dataframe-excel, dataframe-jdbc, dataframe-csv). These files provide guidance on module-specific details, public API descriptions, build commands, architecture, and tests, ensuring consistent documentation across the repository.

The problem: every time you start a session, Claude begins with zero memory of our project. It doesn't know how we build, which module does what, our code style, or our conventions. So it guesses — and guesses are often wrong or generic.
CLAUDE.md fixes that. It's a plain Markdown file with project instructions that Claude Code reads automatically at the start of every session. It's basically an onboarding doc — but for the AI. In it we write the things a new teammate would need to know:

• how to build, lint, and run tests (exact commands);

• what each module is and how the code is organized;

• our code style and conventions;

• hard rules ("don't hand-edit generated files", "don't point tests at prod DB").
  Why one file per module? We put a general CLAUDE.md at the repo root, plus a smaller one inside big modules (core, dataframe-jdbc, dataframe-csv, dataframe-json, dataframe-arrow, dataframe-excel). When Claude works on files in, say, dataframe-jdbc/, it loads that module's file too — so it gets focused, relevant context instead of one giant document.
  What we get out of it:

• Consistency — the AI follows our conventions instead of inventing its own each time.

• Fewer mistakes — it runs the right commands and doesn't touch things it shouldn't.

• Less hand-holding — you don't have to re-explain the project in every chat.

It's just Markdown, in git — anyone can read and edit it in a PR. No special tooling.
In short: these files are how we teach the AI our project once, so it's useful from the first message instead of guessing.

[Copilot]

Pull request overview

This PR adds module-scoped CLAUDE.md documentation across the Kotlin DataFrame repository to complement the root CLAUDE.md, providing contributors with consistent, module-specific guidance (APIs, architecture notes, build/test commands, and testing layouts).

Changes:

• Add new CLAUDE.md files for core and key I/O modules (dataframe-json, dataframe-arrow, dataframe-excel, dataframe-jdbc, dataframe-csv).
• Expand dataframe-arrow/README.md with a clearer description of Arrow IPC/Feather and Parquet (read-only) support.
• Add detailed public API and testing notes per module, including binary-compat guidance where applicable.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
CLAUDE.md	Adds repository-wide contributor guidance (build/test, architecture, KoDEx, constraints).
core/CLAUDE.md	Documents core module architecture, key types, API→impl conventions, codegen, and build specifics.
dataframe-json/CLAUDE.md	Documents JSON module purpose, public IO API, inference knobs, and tests.
dataframe-arrow/README.md	Clarifies Arrow IPC/Feather read/write support and Parquet read-only support.
dataframe-arrow/CLAUDE.md	Documents Arrow module API surface, implementation map, JVM flags, and tests.
dataframe-excel/CLAUDE.md	Documents Excel read/write API surface, dependencies, stability expectations, and tests.
dataframe-jdbc/CLAUDE.md	Documents JDBC module API, DbType pipeline/extension points, and test organization.
dataframe-csv/CLAUDE.md	Documents CSV/TSV/delimited IO API layout, implementation split, and tests/benchmarks.

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[Jolanrensen]

hallucination. There's no kt files in that folder

[Jolanrensen]

please rename them all to AGENTS.md and also add once in /docs and/or /samples

[Jolanrensen]

also I/O modules but experimental

[Jolanrensen]

Column(s) Selection DSL

[Jolanrensen]

Overall good additions! See comments

[Jolanrensen]

I don't think these are needed, right?

[Jolanrensen]

plus it seems some places no longer have CLAUDE.md files and others have been turned into this. I would only keep AGENTS.md files and not clutter stuff with agent-specific files.

[zaleslaw]

No, you need, for now it's officially recommended practice (to have empty Claude.md with reference to Agents.md) it significantly reduces the number of ignoring these files from the Claude agent side

[Jolanrensen]

Okay, good :)

It seems there's still some modules lacking AGENTS.md files:

• dataframe-openapi(-generator)
• dataframe-jupyter (!)
• dataframe-geo(-jupyter)
• common-test-utils

and maybe build(-settings)-logic, but those also have a comprehensive README.md already