v0.2.1 — README install rewrite (Claude Code + CODEX)

[0.2.1] — 2026-05-27

Documentation-only patch on top of 0.2.0. No code changes.

Changed

• README.md rewritten to lead with install instructions for Claude
  Code + OpenAI CODEX (the two MCP-aware coding agents this plugin
  primarily targets), each with a concrete .mcp.json /
  ~/.codex/config.toml snippet plus a one-shot copy/paste invocation.
• Removed the broken curl … releases/download/v0.2.0/… instruction.
  The v0.2.0 release was crates.io-only — no GitHub release artifact
  existed for that tag, so the documented URL returned HTTP 404.
  Pre-built Linux x86_64 binary still ships at the v0.1.0 GH release;
  documented as a fallback with the honest caveat that it's stale vs
  the published crate.
• Install section reordered: cargo install heliosdb-codekb-mcp is
  now the only first-class recommendation. Pre-built binary is
  demoted to "alternative" with the version-staleness warning.