feat: opencode zen adapter + first-run config generation, fix stats/stream bugs

- adapters/opencode.lua: opencode.ai zen free pool adapter — sends the
  opencode client User-Agent (zen fingerprints clients by UA; non-official
  clients hit FreeUsageLimitError); pairs with api_key: public
- config: no config file ships in the repo; first run generates a default
  config at the -config path with a random admin key, loopback listen and a
  keyless zen source (config.EnsureDefault); remove config.example.yaml
- lua: seed bundled adapters from the embedded FS instead of a hardcoded
  name list
- ui: widen model kind select (chat was clipped to 'cha')
- phase 5 bugfixes: stats ms/s bucket mixing, cleanScopes nil, ctx.Err
  guards, direct-path ModelAvailable, empty stream body failure,
  bestImageModel rewrite, transform failure recording, Core.mu, timer,
  effective model for tool-calls

internal/gateway/ui/index.html

@@ -205,7 +205,7 @@ label{display:block;font-size:12px;color:var(--muted);margin:12px 0 5px}
 .row{display:flex;gap:12px}.row>div{flex:1}
 .model-row{display:flex;gap:6px;align-items:center;width:100%}
 .model-row .m-id{flex:1;min-width:0;width:0}
-.model-row .m-kind{flex:0 0 70px;width:70px}
+.model-row .m-kind{flex:0 0 96px;width:96px}
 .model-row .del{flex:0 0 auto;padding:4px 8px}
 .muted{color:var(--muted)}
 .hidden,.hidden#tab-chat{display:none}