docs: index agent guidance sources

[rianjs]

Summary

• add AGENTS.md as a peer Codex entrypoint
• shrink CLAUDE.md into a thin source index
• move repo-local development facts into docs/development.md while linking shared standards and automation to their canonical repos

Verification

• git diff --check
• rg -n -F "Source of truth:" AGENTS.md CLAUDE.md docs/development.md
• rg -n -F "Local convenience copy, if present:" AGENTS.md CLAUDE.md docs/development.md
• rg peer-entrypoint checks for AGENTS/CLAUDE cross-dependencies
• rg duplication guard for shared CI/release mechanics
• make check

Closes #5

[rianjs]

Major
docs/development.md:13 preserves stale repo-local guidance: “The only Go code today is internal/placeholder.” The current tree has cmd/cr/main.go, cmd/cr/main_test.go, and internal/version/version.go, so the new canonical repo-local guide would be factually wrong. Update the overview to describe the scaffold cmd/cr binary and version package, or remove the “only Go code” claim.

Minor
docs/development.md says “Use this repository for shared action and reusable workflow implementations” under Shared Automation. In this file, “this repository” reads as codereview-cli, not open-cli-collective/.github. Reword to “Use open-cli-collective/.github…” or similar.

[rianjs]

No findings.

The updated diff resolves the stale scaffold fact and the ambiguous automation wording. The guidance now stays thin, keeps AGENTS.md and CLAUDE.md as peer entrypoints, moves repo-local facts into docs/development.md, and links shared standards/automation through the established source/local pattern without restating CI or release mechanics.