docs: use renamed rfc2119 npm package in AGENTS.md

[tylerwillis]

AGENTS.md still told agents to run npx 2119 — that package doesn't exist on npm (the published package is rfc2119), and the stale name is squattable. All three invocations (lint, check, review) now use npx rfc2119.

Authored by a Panopticon fleet agent (github-peer-reviewed workflow); pushed from the host because the task container's GitHub token lacks contents-write. Note for review: the agent couldn't run npx rfc2119 check in-container (no Node in the runner image); CI will be the first real check run.

Please merge with a regular merge commit, not squash.

🤖 Generated with Claude Code

https://claude.ai/code/session_014eX855AM81mKyj2bM1dBp1

[tylerwillis]

Closing after verification: the package is named rfc2119 but its executable bin is literally 2119 (package.json bin: {"2119": "dist/cli.js"}) — so npx 2119 is the correct repo-local invocation and the original docs were right. npx rfc2119 also resolves via the package name, but documenting the actual binary is clearer.