fix(web): make checkout switching reliable

[badcuban]

Problem

Changing a thread checkout after a completed turn could leave the session stuck in Preparing turn. Composer and Source could disagree, and inferred subagent worktrees could overwrite an explicit user choice.

Fix

Route every checkout selector through one explicit command. Show the selected checkout immediately, preserve active execution until it can move safely, prevent stale subagent inference from overriding the selection, and only preserve starting state when a real turn is pending.

Verification

• vp fmt
• vp lint
• vp run typecheck
• Affected contract, decider, provider runtime, store, and Source checkout tests passed
• Full browser run passed 489/491; the two unchanged-component failures passed when rerun alone
• Independent production review approved the final diff

[vercel]

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
threadlines	Skipped		Aug 24, 2026 9:24pm