fix(#4763): surface the displaced session record and pass --phase from the executor decision loop (#4919)

* test(#4763): failing-first — replaced-record payload and executor --phase pins

* fix(#4763): surface the displaced session record and pass --phase from the executor decision loop

state record-session keeps its last-writer-wins write (the recorded single-slot
handoff design) but no longer displaces silently: when a non-empty Stopped At or
authored Resume File record is replaced, the payload carries the full prior text
under replacedRecord. Same-value rewrites, the insert path, and the #944
template-default DWIM are not displacements and report nothing.

The executor decision loop now passes --phase "${PHASE}" to state.add-decision,
matching execute-plan.md, so decisions stop inheriting whichever phase the
global pointer names (#4763 case 2). advance-plan is unchanged (#3311 by-design).

Emitted-Drift-Ack-Growth: gsd-executor.md — the decision loop gained its --phase guard and a comment naming why (#4763)

* docs(#4763): add the changeset fragment

* docs(#4763): backfill the changeset PR number

---------

Co-authored-by: sim <sim@local>

src/state.cts

@@ -1961,11 +1961,32 @@ function cmdStateRecordSession(cwd: string, options: StateRecordSessionOptions,
   const updated: string[] = [];
   let sessionCreated = false;
   const divergedFields: string[] = [];
+  // #4763 (1): last-writer-wins stays (the recorded single-slot handoff design),
+  // but a displaced record is no longer silent. The pre-write session record is
+  // captured below and surfaced in the payload under `replacedRecord` whenever a
+  // replacement actually changes a non-empty prior value.
+  const priorRecord: { stoppedAt?: string; resumeFile?: string } = {};
   // ADR-3473 §8.7 (#3872): caller-allocated out-param, filled with the
   // transaction's own pre-write snapshot + body by `applyPostSyncPreservation`.
   const preWriteState: StatePreWriteSnapshot = {};
 
   readModifyWriteStateMd(statePath, (content) => {
+    // #4763 (1): read the pre-write session record. The capture mirrors the
+    // WRITER, not the snapshot reader: stateReplaceField replaces the FIRST
+    // case-insensitive label match anywhere in the document, so the capture is
+    // document-wide too — scoping it to ## Session would miss an archive-section
+    // line the writer actually displaced. Continuation lines join via
+    // stateFieldContinuation (the sanctioned joiner — stateExtractField alone is
+    // first-line-only), so a wrapped multi-line handoff is surfaced whole.
+    const capturePrior = (fieldName: string): string | undefined => {
+      const first = stateExtractField(content, fieldName);
+      if (first === null) return undefined;
+      const cont = stateFieldContinuation(content, fieldName);
+      return cont ? `${first}\n${cont}` : first;
+    };
+    priorRecord.stoppedAt = capturePrior('Stopped At');
+    priorRecord.resumeFile = capturePrior('Resume File');
+
     // Update Last session / Last Date
     let result = stateReplaceField(content, 'Last session', now);
     if (result) { content = result; updated.push('Last session'); }
@@ -2181,6 +2202,27 @@ function cmdStateRecordSession(cwd: string, options: StateRecordSessionOptions,
   if (reconciledUpdated.length > 0) {
     const result: Record<string, unknown> = { recorded: true, updated: reconciledUpdated };
     if (sessionCreated) result['created'] = true;
+    // #4763 (1): surface any non-empty prior record the write displaced. Gated
+    // on reconciledUpdated (the fields that actually persisted, post-#3957
+    // reconcile) rather than the pre-reconciliation updated[]. Only fields with
+    // real prior content differing from the caller's value count — same-value
+    // rewrites, the insert path (no prior label), and the #944 template-default
+    // DWIM (defaults match case-insensitively, so a case-variant 'none' →
+    // 'None' rewrite is normalization, not displacement) are excluded.
+    const isResumeTemplateDefault = priorRecord.resumeFile !== undefined
+      && KNOWN_TEMPLATE_DEFAULTS['Resume File'].some(
+        (d) => d.toLowerCase() === priorRecord.resumeFile?.toLowerCase());
+    const replacedRecord: Record<string, string> = {};
+    if (reconciledUpdated.includes('Stopped At') && priorRecord.stoppedAt
+        && priorRecord.stoppedAt !== options.stopped_at) {
+      replacedRecord['Stopped At'] = priorRecord.stoppedAt;
+    }
+    if (reconciledUpdated.includes('Resume File') && priorRecord.resumeFile
+        && !isResumeTemplateDefault
+        && priorRecord.resumeFile !== (options.resume_file ?? undefined)) {
+      replacedRecord['Resume File'] = priorRecord.resumeFile;
+    }
+    if (Object.keys(replacedRecord).length > 0) result['replacedRecord'] = replacedRecord;
     output(result, raw, 'true');
   } else if (updated.length === 0) {
     // Nothing was ever attempted — no --stopped-at/--resume-file supplied