docs: rewrite README; add architecture diagram

Rewrite the package README in the style of the MCP Python SDK, with
sections for envelopes, sessions, jobs, tools + JobContext, streams,
subscriptions, leases, artifacts, human-in-the-loop, conformance and
examples. Add an architecture diagram (mermaid source + rendered SVG)
at docs/images/.

Point the RFC-0001-v2.md pointer at the renamed
spec/docs/draft-arcp-01.md (IETF Internet-Draft convention now used in
the spec repo).

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

Author: nficano