fix(server): open existing chat instead of failing on duplicate bootstrap thread.create

[enisze]

Problem

Sending the first message on a local draft thread embeds a bootstrap.createThread in the thread.turn.start command, using a threadId the client pre-generated. The server expands that into a real thread.create. If that threadId already exists server-side (a double-send, a retry, or a draft that was already promoted), the requireThreadAbsent invariant fires, the whole bootstrap turn fails, and the user sees an error banner:

Orchestration command invariant failed (thread.create): Thread '…' already exists and cannot be created twice.

Fix

Bootstrapping a draft is inherently a create-if-absent operation, so the duplicate thread.create should be a no-op rather than a hard failure. The bootstrap path now catches that specific invariant, treats it as a no-op, and continues the turn against the existing thread. The message lands, the thread starts, and the draft route's existing navigation logic moves the user into the already-created chat — instead of showing the error.

Done server-side because the error only reaches the client as a generic OrchestrationDispatchCommandError message string; detecting it in the UI would mean fragile string matching and would drop the user's message.

Changes

• commandInvariants.ts — extracted the "already exists" detail into a shared function and added isThreadAlreadyExistsInvariantError(error, threadId), a precise predicate (matches on both commandType === "thread.create" and the exact detail, so it never swallows a different invariant like a missing project).
• ws.ts — the bootstrap thread.create dispatch catches OrchestrationCommandInvariantError, treats the "already exists" case as a no-op, and keeps createdThread = false so failure-cleanup never deletes a pre-existing thread.
• commandInvariants.test.ts — coverage for the new predicate, including negative cases.

Verification

• pnpm typecheck — clean
• Orchestration suite: 191/191 passing, including the new predicate test

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