fix(#2567): scope Paused At to ## Session + guard Last Activity date regression (#2660)

* test(#2567): add failing regression for stale state field overwrites

buildStateFrontmatter extracts Last Activity and Paused At from the full
STATE.md body via stateExtractField, which matches the first 'Field:' line
anywhere. Historical archive sections containing stale field-shaped lines
silently overwrite the correct frontmatter value on every sync, and because
the poisoning line stays in the body it regresses on the next write.

Same divergence class as Bug #2444 (which scoped Stopped At to ## Session
but did not propagate). Failing-first: all three tests reproduce the bug
on unmodified next (verified via the dedicated red run on the test-only
commit).

* fix(#2567): scope Paused At to ## Session + guard Last Activity date

Two complementary fixes for the stale-archive-overwrites-frontmatter bug
class, chosen per field semantics:

- Paused At is a session field: scope extraction to ## Session (via the
  existing matchSessionSection helper), exactly mirroring the #2444 fix for
  Stopped At. A stale 'Paused At:' line in an archive section can no longer
  win over the current value. Falls back to full body when no ## Session.

- Last Activity has no single canonical section (it appears in the
  preamble, ## Configuration, and ## Current Position across STATE.md
  layouts), so a section scope cannot reliably exclude archive copies.
  Instead guard the information-losing direction: when the body-derived
  date is OLDER than the existing frontmatter date, keep the existing
  value and description (preferNewerLastActivity). Applied at both the
  write seam (syncStateFrontmatter) and the read seam (cmdStateJson) so
  they agree. Non-date values pass through unchanged.

A first attempt scoped ALL current-state fields to the body preamble, but
that broke STATE.md variants where the fields legitimately live inside
## Configuration / ## Current Position (regressed 4 frontmatter.test.cjs
suites). This minimal fix targets only the two fields the issue names.

* docs(#2567): add changeset fragment for stale state field overwrite fix

* docs(#2567): backfill PR number in changeset fragment

src/state.cts

@@ -1298,6 +1298,39 @@ function matchSessionSection(body: string): string | null {
   return section ? section.body : null;
 }
 
+/**
+ * #2567: prevent a stale archive "Last activity:" line from overwriting a
+ * newer frontmatter value. `stateExtractField` matches the first body
+ * occurrence, which may be a historical line in an archive section. Unlike
+ * Stopped At / Paused At (which canonically live in `## Session`), Last
+ * Activity has no single canonical section — it appears in the preamble,
+ * `## Configuration`, and `## Current Position` across STATE.md layouts, so a
+ * section scope cannot reliably exclude archive copies. Guard the
+ * information-losing direction instead: when the body-derived date is OLDER
+ * than the existing frontmatter date, keep the existing value and its
+ * description. Applied at both the write seam (syncStateFrontmatter) and the
+ * read seam (cmdStateJson) so they agree. Date fields only — non-date values
+ * pass through unchanged.
+ */
+function preferNewerLastActivity(
+  existingFm: Record<string, unknown> | null,
+  derivedFm: Record<string, unknown>,
+): void {
+  if (!existingFm) return;
+  const exRaw = existingFm['last_activity'];
+  const derRaw = derivedFm['last_activity'];
+  if (typeof exRaw !== 'string' || typeof derRaw !== 'string') return;
+  const exDate = exRaw.slice(0, 10);
+  const derDate = derRaw.slice(0, 10);
+  if (!/^\d{4}-\d{2}-\d{2}$/.test(exDate) || !/^\d{4}-\d{2}-\d{2}$/.test(derDate)) return;
+  if (derDate < exDate) {
+    derivedFm['last_activity'] = exRaw;
+    if (existingFm['last_activity_desc'] !== undefined) {
+      derivedFm['last_activity_desc'] = existingFm['last_activity_desc'];
+    }
+  }
+}
+
 function parseProsePhaseField(value: string | null): { phase: string | null; name: string | null } {
   // #2121 Phase 2 (#2125): delegate to the canonical anchored parser so this
   // module holds no independent prose phase-id regex. Drives #2111 — the
@@ -1520,8 +1553,8 @@ function buildStateFrontmatter(bodyContent: string, cwd: string | undefined): Re
   const proseLastActivity = parseProseLastActivityField(rawLastActivity);
   const lastActivity = proseLastActivity.date ?? rawLastActivity;
   const lastActivityDesc = stateExtractField(bodyContent, 'Last Activity Description') ?? proseLastActivity.description;
-  // Bug #2444: scope Stopped At extraction to the ## Session section so that
-  // historical "Stopped at:" prose elsewhere in the body (e.g. in a
+  // Bug #2444 / #2567: scope Stopped At AND Paused At extraction to the
+  // ## Session section so historical prose elsewhere in the body (e.g. in a
   // Session Continuity Archive section) never overwrites the current value.
   // Fall back to full-body search only when no ## Session section exists.
   // #1101: prefer the canonical `## Session` block, falling back to the bootstrap
@@ -1529,7 +1562,9 @@ function buildStateFrontmatter(bodyContent: string, cwd: string | undefined): Re
   const sessionSectionMatch = matchSessionSection(bodyContent);
   const sessionBodyScope = sessionSectionMatch ?? bodyContent;
   const stoppedAt = stateExtractField(sessionBodyScope, 'Stopped At') || stateExtractField(sessionBodyScope, 'Stopped at');
-  const pausedAt = stateExtractField(bodyContent, 'Paused At');
+  // #2567: Paused At is a session field — scope it to ## Session too so a
+  // stale "Paused At:" line in an archive section cannot overwrite the value.
+  const pausedAt = stateExtractField(sessionBodyScope, 'Paused At');
 
   let milestone: string | null = null;
   let milestoneName: string | null = null;
@@ -1822,6 +1857,10 @@ function syncStateFrontmatter(content: string, cwd: string | undefined): string
     }
   }
 
+  // #2567: guard the information-losing direction — a stale archive
+  // "Last activity:" line must not overwrite a newer frontmatter value.
+  preferNewerLastActivity(existingFm, derivedFm);
+
   const yamlStr = reconstructFrontmatter(derivedFm as unknown as Frontmatter);
   return `---\n${yamlStr}\n---\n\n${body}`;
 }
@@ -2228,6 +2267,11 @@ function cmdStateJson(cwd: string, raw: boolean): void {
     built['progress'] = normalizeProgressNumbers(existingFm['progress']);
   }
 
+  // #2567: guard the information-losing direction — a stale archive
+  // "Last activity:" line must not surface as the current value. Mirrors the
+  // syncStateFrontmatter guard so the read path agrees with the write path.
+  preferNewerLastActivity(existingFm, built);
+
   output(built, raw, JSON.stringify(built, null, 2));
 }