v0.21.0 — opencode as a first-class cross-AI tool

Changed

• opencode is a first-class cross-AI tool in the connect docs. De-staled the connect/install prose and the KNOWN_TOOLS comment that still enumerated only "Codex, Gemini, Cursor" — opencode (and the other adapters) are auto-detected and wired by captain-memo connect; the help text and comments now reflect that.