Normalize cross-repo links to absolute GitHub URLs

Each SDK is its own GitHub repository, so relative paths like
../spec/docs/draft-arcp-1.1.md don't resolve when the README is
viewed on github.com. Switching to absolute spec URLs makes the
links work in both the GitHub UI and any rendered docs surface.
A few stale references to removed files (RFC-0001-v2.md, PLAN.md)
and snake_case sample dirs are tidied along the way.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nficano

README.md

@@ -1,7 +1,7 @@
 # arcp
 
 Ruby SDK for ARCP v1. The wire protocol is specified in
-[../spec/docs/draft-arcp-1.1.md](../spec/docs/draft-arcp-1.1.md). This
+[the ARCP spec](https://github.com/agentruntimecontrolprotocol/spec/blob/main/docs/draft-arcp-1.1.md). This
 gem implements the full v1 envelope, session handshake, job lifecycle,
 event stream, leases, and error model.