fix: start draft sessions from default model/agent and honor OpenCode default_agent

A new draft session inherited the previous session's model/agent instead of
resetting to defaults, because opening a draft restored the directory snapshot
without re-applying the startup default cascade. When the prior session ran in
a worktree, defaults were resolved against the worktree directory's provider
list, which omits project/global-scoped providers, so the default agent's model
fell back to opencode/big-pickle.

Resolve the default agent/model via a shared cascade (settings default ->
OpenCode default_agent -> build -> first), resolve the model from the agent's
pinned model/variant or OpenCode's config model, and activate the project's
config (not the worktree's) when opening a draft.

packages/ui/src/sync/session-ui-store.ts

@@ -617,7 +617,16 @@ export const useSessionUIStore = create<SessionUIState>()((set, get) => ({
       useInputStore.getState().setPendingInputText(options.initialPrompt)
     }
 
-    void activateConfigForDirectory(directory)
+    // Config (providers/agents/default model+agent) lives at the PROJECT level. When the user
+    // came from a worktree session, `directory` is the worktree path, whose provider list does
+    // not include project/global-scoped providers (e.g. the default agent's non-opencode model)
+    // — resolving defaults against it would wrongly fall back to opencode/big-pickle. Activate
+    // the project's config instead so the default cascade matches app startup, then re-apply it
+    // (a fresh draft must start from defaults, not inherit the previous session's selection).
+    const configDirectory = normalizePath(selectedProject?.path ?? null) ?? directory
+    void activateConfigForDirectory(configDirectory).then(() => {
+      useConfigStore.getState().applyDefaultModelAgentSelection()
+    })
   },
 
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