Rewind and diff work past a run that never started; skip the files question when nothing changed

[ziyadakl]

Summary

A run that never starts (cancelled while queued, or steered into the run before it) gets no checkpoint. Rewind and the diff panel both looked up a checkpoint by exact run number, so the gap broke them. Both now use the newest ready checkpoint at or before that run (shared: packages/shared/src/checkpointOrdinal.ts).

 revertThreadCheckpoint(turnCount)            # packages/client-runtime/src/operations/commands.ts
-  checkpoints.findLast(c => c.appRunOrdinal === turnCount)
+  checkpointAtRunOrdinal(checkpoints, turnCount)   # newest at or before, else thread start

 CheckpointDiffQuery "from" checkpoint        # apps/server/src/checkpointing/CheckpointDiffQuery.ts
-  exact run number
+  checkpointAtRunOrdinal(...)

The "Edit from here?" dialog no longer asks about files when no files changed since the revert point:

 onRevertToTurnCount(turnCount)               # apps/web/src/components/ChatView.tsx
-  open "Edit from here?"
+  if revertTouchesFiles(turnDiffSummaries, turnCount)   # apps/web/src/session-logic.ts
+    open "Edit from here?"
+  else
+    rewind now, keep files, prompt back in composer

Evidence

Real thread: run 1 completed, run 2 cancelled before start (no checkpoint), run 3 completed. Rewind on run 3's message failed.

• Before: commands.test.ts:280 → OrchestrationV2CheckpointUnavailableError: Rollback target run ordinal 2 is unavailable for thread thread-v2.
  After: Tests 28 passed (28)
• Before: CheckpointDiffQuery.test.ts:199 → CheckpointRefUnavailableError: ... Checkpoint ref is unavailable for turn 2.
  After: Test Files 2 passed (2), Tests 34 passed (34)
• Before: session-logic.test.ts:348 → TypeError: revertTouchesFiles is not a function
  After: Tests 51 passed (51)

Typecheck: packages/shared, packages/client-runtime, apps/server, apps/web exit 0.

Merge Danger

Door: two-way

Lookup and UI logic only; no data migration or stored-state change. Revert the two commits to undo.

Blast Radius: small

Touches rewind and the diff panel only. One edge: an empty changed-file list can also mean the server could not compute the diff, so the dialog is skipped and files are kept. Nothing is lost.

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