Reciprocal pointers so humans landing in CONTRIBUTING.md discover
the agent/operational conventions in AGENTS.md, and agents landing
in AGENTS.md discover the CLA gate and pre-PR checks. Adds a short
Development section (test + fmt commands) and a Before-opening-a-PR
checklist to CONTRIBUTING.md; CLA paragraph preserved verbatim and
moved to its own section.
Cross-vendor agent doc with the project-specific bits not derivable
from README or code: test filter env vars, the leak-detection test
invariant, the exact zig fmt CI command, and a pointer to mirror
neighboring file conventions for @import alias case and struct-init
inference. CLAUDE.md is a one-line @AGENTS.md import shim for Claude
Code, which doesn't read AGENTS.md natively.
