fix: retain first turn messages and restore last user message to composer on revert

[shoaib050326]

Summary

Fixes two issues when clicking "Revert to this point" on a single-turn conversation:

1. Blank screen: The revert turn count was computed as checkpointTurnCount - 1 = 0, but the checkpoint filter (checkpointTurnCount <= 0) matched no checkpoints (first checkpoint is at checkpointTurnCount = 1), causing all messages to be removed.

2. Lost input text: After revert completed, the composer input was not populated with the last retained user message text.

Changes

• apps/web/src/store.ts — Use Math.max(1, turnCount) as the effective filter threshold in the "thread.reverted" reducer so the first checkpoint is always retained.
• apps/server/src/orchestration/projector.ts — Same fix in the server-side projector for consistency.
• apps/web/src/components/ChatView.tsx — After revert completes, restore the last retained user message text into the composer input.
• apps/web/src/store.test.ts — Regression test for turnCount: 0 revert.

Verification

• apps/web: typecheck ✓, 90 test files / 926 tests pass ✓
• apps/server: 3 test files / 41 orchestration tests pass ✓

Closes #2268

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Note

Medium Risk
Touches both server and client thread.reverted projection/reducer logic, which can affect message retention during revert (potentially user-visible data loss if incorrect). Scope is small and covered by a new regression test for turnCount=0.

Overview
Fixes single-turn thread revert behavior. When handling thread.reverted, both the server projector and web store now clamp the effective turnCount to at least 1, ensuring the first checkpoint/turn diff and its related messages/plans/activities aren’t wiped when the UI computes turnCount=0.

Improves UX after revert. After a successful revert, ChatView reloads the updated thread from the store and, if the composer is empty, restores the last retained user message text into the composer draft. A regression test was added to verify that reverting with turnCount: 0 preserves first-turn state.

^{Reviewed by Cursor Bugbot for commit 2b75e80. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Fix thread revert to retain first-turn messages and restore last user message to composer

• Clamps turnCount to a minimum of 1 when handling thread.reverted events in both store.ts and projector.ts, preventing a turnCount <= 0 from clearing the first turn's messages, plans, activities, and diff summaries.
• After reverting to a checkpoint in ChatView.tsx, if the composer prompt is empty, it is auto-filled with the text of the last user message from the updated thread.

^{Macroscope summarized 2b75e80.}

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[chatgpt-codex-connector]

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[macroscopeapp]

Approvability

Verdict: Needs human review

1 blocking correctness issue found. Unresolved review comments identify potential bugs: a race condition where the composer reads stale state after revert, and a semantic change to turnCount=0 handling that may cause client/server divergence. These substantive issues warrant human review.

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