fix: Fix model selection lost during task creation

[charlesvien]

Problem

Model selected in the new task input was silently dropped. The session always started with the default model because selection was applied as a post start config patch that raced with the first prompt.

Closes #1380

Closes #1340

Changes

1. Pass selected model through the start session schema all the way to the SDK
2. Fetch model config in parallel with SDK initialization for new sessions
3. Remove post start config patching that raced with first prompt delivery
4. Limit useSessionConnection to reconnects only, new sessions handled by task creation saga

How did you test this?

Manually

[charlesvien]

• feat: Replace preview session process spawn with lightweight config query #1438
• fix: Fix model selection lost during task creation #1437 👈 (View in Graphite)
• fix: Persist new task draft across navigation #1436
• main

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[jonathanlab]

Merge activity

• Apr 2, 11:50 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Apr 2, 12:12 PM UTC: Graphite couldn't merge this PR because it failed for an unknown reason (GitHub is reporting that this PR is not mergeable, despite passing required status checks defined by your branch protection rules. Please check your rulesets for additional blocking criteria. Graphite Merge Queue does not currently support rulesets. Please contact Graphite support for further assistance.).