v0.18.0 — OpenCode Go session header

Fixed

• OpenCode Go was unreachable. The Go endpoints now require every request to carry a stable x-opencode-session header so the gateway can route it and reuse its prompt cache; agent-cli sent none, so every Go request came back HTTP 400 MissingSessionID before a model was reached — both wire formats, every model, with a valid key. Cloud requests now carry a ses_<ulid> minted per agent process (stable across a conversation, distinct between agents, pinnable with [provider.opencode] session_id), plus the User-Agent: agent-cli/<version> OpenCode asks clients to identify themselves with. Local mode is unchanged and sends neither.
• The opencode-go shortcut pointed at a model the Go endpoint does not serve. Go's catalogue is open-weight models only — claude-* is on .../zen/v1, not .../zen/go/v1 — so the defaults now resolve to api = "openai" and a served model instead of api = "anthropic" / claude-sonnet-4-5. Configs that set api or model explicitly are unaffected; those that did not could not complete a turn.
• A model the endpoint does not serve was reported as a revoked API key. ModelError arrives with HTTP 401, which hit the key hint; it now points at GET {base_url}/models, and a missing session id explains itself instead of arriving hintless.
  • Verified against a v0.17.0 binary as the control: the config that fails there with 400 MissingSessionID completes a full five-stage selftest on both wire formats here, and a bare kind = "opencode-go" config works with no edits. Non-Go Zen and local mode produce identical output on both binaries.
  • Docs updated: README.md, README_ja.md, doc/providers/opencode.md, doc/config.md, doc/troubleshooting.md, example/config.example.toml.