Problem

When a follow-up message arrives in an existing Slack thread started by a different user, the requester attribution for model-generated content (such as the Action taken on behalf of line in PR bodies) would be wrong — showing the original thread starter instead of the person who actually made the request.

Root Cause

The session log projection stores pi_message entries as-is, including any runtime turn context blocks (<runtime-turn-context>...<requester>...</requester>...) that were embedded in user messages from prior turns.

In loadPiMessagesForTurn, the active-turn resume path correctly strips old runtime context before returning:

return {
  piMessages: stripRuntimeTurnContext(
    trimTrailingAssistantMessages(sessionRecord.piMessages),
  ),
};

But the projection path did not strip:

return {
  canCompact: true,
  piMessages: projection,   // ← no strip — bug
};

When hasRuntimeTurnContext(priorPiMessages) found the old context from user A's session, it returned true, so needsBootstrapContext = false and user B's identity was never injected into the prompt. The model then used user A's stale <requester> block for any attributions it generated.

Note: git commit co-author attribution was unaffected — that path uses the per-turn resolved identity directly rather than the model prompt.

Fix

Apply stripRuntimeTurnContext to the projection path, making it consistent with the active-turn resume path:

return {
  canCompact: true,
  piMessages: stripRuntimeTurnContext(projection),
};

Context compaction already calls stripRuntimeTurnContext internally, so no double-strip concern.

Verification

• tsc --noEmit clean
• New regression test: projection with user A's embedded requester context does not suppress injection of user B's requester context after stripping
• 3 tests pass (respond-helpers-runtime-context.test.ts)

Remaining Risk

Other consumers of loadProjection that check hasRuntimeTurnContext on the result could have the same class of bug, though the main model-input path flows through loadPiMessagesForTurn. Compaction is already safe (strips internally).