fix: correct OpenCode 2 command transport

[ByBrawe]

Summary

• fix the OpenCode 2 command transport introduced in feat: add OpenCode 2 command loop actions #96: the real @opencode-ai/plugin@next wrapper treats id as an optional msg_... message ID, not the slash-command name
• send command names through the verified command field instead, using { sessionID, command, arguments? }
• update the host contract, runtime adapter, and idle-safe prompt/command scheduler to use the corrected request shape and reject the old { id: "review" } shape
• omit id entirely from command-loop dispatches
• strengthen command runtime, host-contract, and adapter/composition tests with exact request-shape assertions
• strengthen the real OpenCode 2 adapter probe so it creates a real session and invokes an intentionally missing command through the command field; the official gate requires the host error Command not found: <sentinel>, which would fail differently if id were used again
• real-host probing also confirmed that session.command({ command: "compact" }) returns Command not found: compact, and the empty command registry contains no compact entry, so /compact remains deliberately unsupported in the V2 runtime
• shell actions also remain deliberately unsupported because the real plugin context currently exposes no session.shell

Branch validation passed syntax, V2 host/command/adapter contracts, the complete existing test suite, git diff --check, and the real @opencode-ai/plugin@next command-field probe. Temporary validation workflow was removed before this PR. Package version remains 0.5.28.

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