revert: stop forwarding the Small Model override into the managed OpenCode config

Reverts #2687. In real use the injected small_model behaves poorly with
OpenCode: its internal small-model consumers and OpenChamber's own small
model are different things and must stay configured separately.

packages/web/server/lib/small-model/DOCUMENTATION.md

@@ -137,20 +137,6 @@ other runtime API.
 - `routes.js` — `GET /api/small-model` (resolution preview) and
   `POST /api/small-model/generate` (`{ prompt, system?, maxOutputTokens?,
   model?, directory? }` → `{ text, providerID, modelID, source }`).
-- `config-injection.js` — applies the Settings → Chat → Small Model override
-  to the config injected into the **managed OpenCode process**
-  (`OPENCODE_CONFIG_CONTENT`), so OpenCode's own internal `small_model`
-  consumers — session title and summary generation — use the user's explicit
-  choice instead of OpenCode's fallback chain. Only an explicit override
-  (`smallModelUseDefault === false` with a non-empty `smallModelOverride`) is
-  injected; "use default" leaves the config untouched so OpenCode's own
-  resolution stays authoritative. Wired into `getManagedOpenCodeEnv` in
-  `server/index.js`; the pure helper is unit-tested in
-  `config-injection.test.js`. External OpenCode servers are unaffected (they
-  are not launched with this env). The injected `small_model` is baked into
-  `OPENCODE_CONFIG_CONTENT` when the managed process spawns, so changing the
-  override in Settings applies on the next managed OpenCode restart, not to
-  the process already running.
 
 ## Which providers the pickers may offer