Move the comparison to COMPARISON.md and give OpenSpec its own section

[emafriedrich]

What

• New COMPARISON.md holding the "How it compares" content; the README section is now a single link to it.
• OpenSpec gets its own section instead of being lumped in with Spec Kit and Kiro. The old line ("those drive one task from a spec") was wrong for OpenSpec, which also keeps living specs in the repository.
• Opens with the positioning: this is not spec-driven development.
• Cursor rules moved next to ADRs + AGENTS.md (instructions, not specs). BMAD added to the spec-driven tools.
• The OKF paragraph moved over unchanged.

Notes for review

• The OpenSpec table is dated "As of October 2026". Its claims come from the OpenSpec repository, discussion #169 and third-party write-ups, not from running the tool.
• Docs only; no behavior change.