Extract project conventions into AGENTS.md; slim CLAUDE.md

[tatiana]

Summary

Make the project's agent guidance reusable across AI coding tools:

• Move every agent-neutral section of CLAUDE.md (commands, architecture, coding standards, commit-message format, documentation style, etc.) into a new top-level AGENTS.md. The content is byte-identical apart from the introductory paragraph, which is rewritten in agent-neutral language and names CLAUDE.md as the canonical place for Claude-only overrides.
• Slim CLAUDE.md to a small pointer that defers to AGENTS.md. The pointer remains the file Claude Code reads on session start, but it no longer duplicates project conventions. Claude-specific instructions can accumulate there in the future without diverging from the shared baseline.
• Document the AI-agent attribution convention (originally PR Document the AI agent attribution convention #2718, since folded into this branch): give the assisting agent visible credit via a trailing 🤖 Generated with … marker, never via a Co-Authored-By trailer (that surfaces in the GitHub contributor graph and git shortlog, which should stay human-only).
• Add AGENTS.md to the Check-changed-files allowlist in .github/workflows/test.yml so docs-only edits to AGENTS.md don't trigger the full unit/integration matrix, mirroring how CLAUDE.md is already handled.

This unlocks other agents — Cursor, Codex, Copilot, etc. — using the same conventions Claude already does, without us maintaining parallel copies.

Review feedback addressed

• Copilot: add AGENTS.md to the Check-changed-files allowlist in .github/workflows/test.yml (mirrors the existing CLAUDE.md line).
• Copilot: change British "behaviour" to American "behavior" in the AGENTS.md intro paragraph for consistency with the rest of the document.

Rebased onto main

The original revision was stacked on claude-md-require-precommit (PR #2702). #2702 has since squash-merged to main (commit ea04d0da), so this branch has been rebased onto the current main. The redundant pre-squash commits were dropped during rebase (693c3d9c identified as "patch contents already upstream"; f8a6bf03 skipped manually).

The companion attribution-convention PR #2718 has also been folded into this branch since it built on top of claude-md-extract-agents-md.

Test plan

• pre-commit run --files CLAUDE.md AGENTS.md .github/workflows/test.yml — all applicable hooks pass locally.
• CI run on this PR — pre-commit.ci passes; the full unit/integration matrix runs (because this PR touches .github/workflows/test.yml, which is not — and should not be — on the allowlist). Future docs-only AGENTS.md PRs will skip the matrix.

🤖 Generated with Claude Code (https://claude.com/claude-code)

[Copilot]

Pull request overview

This PR restructures the repository’s AI-agent guidance so that project conventions live in a tool-agnostic document (AGENTS.md), while CLAUDE.md becomes a small Claude Code entrypoint that defers to the shared conventions.

Changes:

• Add AGENTS.md containing the previously shared, agent-neutral guidance (commands, architecture, coding standards, docs style, etc.).
• Replace the bulk of CLAUDE.md with a short pointer to AGENTS.md, reserving CLAUDE.md for future Claude-specific overrides.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
CLAUDE.md	Slimmed to a Claude Code-specific pointer to the shared conventions in AGENTS.md.
AGENTS.md	New shared conventions document intended to be reused across multiple AI coding agents/tools.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.04%. Comparing base (5b8ade7) to head (a77495e).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2703   +/-   ##
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  Coverage   98.04%   98.04%           
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  Files         105      105           
  Lines        7867     7867           
=======================================
  Hits         7713     7713           
  Misses        154      154           

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[pankajkoti]

Not sure of the impacts, but let's try and see.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[tatiana]

Status update on this PR:

• Rebased onto current main to clear the merge conflict caused by #2702 (which this branch was stacked on) squash-merging as ea04d0da. The two redundant pre-squash commits were dropped during rebase; a77495eb ("Document the AI agent attribution convention") was preserved as it had not yet landed elsewhere.
• Addressed both Copilot review items:
  • Added AGENTS.md to the Check-changed-files allowlist in .github/workflows/test.yml (discussion). Mirrors how CLAUDE.md is already handled. Note: the full unit/integration matrix still runs on this PR because the PR touches .github/workflows/test.yml itself, which is not (and should not be) on the allowlist. The allowlist change takes effect for future docs-only AGENTS.md edits.
  • Changed British "behaviour" → American "behavior" in the AGENTS.md intro paragraph for consistency with the rest of the document (discussion).
• pre-commit.ci is now passing (was previously failing on "error during mergeable check", caused by the merge conflict).

@pankajkoti — the previous approval was on the pre-rebase commit hash and has been invalidated by the force-push. Could you take another look?