docs: index agent guidance sources

[rianjs]

Summary

• remove the AGENTS.md to CLAUDE.md dependency pattern
• shrink AGENTS.md and CLAUDE.md into peer source indexes
• move New Relic-specific 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 verify

Closes #126

[rianjs]

No findings.

The PR removes the AGENTS.md -> CLAUDE.md dependency, keeps both entrypoints as peer indexes, preserves nrq-specific facts in docs/development.md, and links shared standards/automation through the established source/local pattern. The shared CI/release mechanics were removed rather than relocated, and the added scriptability.md reference covers the init/set-credential/nrq me guidance.