fix(#3233): no-op state update-progress when the milestone scan finds zero plans (#3375)

* test(#3233): zero plans (0/0) is a no-op; plans-but-none-done still writes 0%

cmdStateUpdateProgress mapped 0/0 through clampPercent to 0% and rewrote the
shipped Progress record after milestone close. Replace the stale 'handles zero
plans gracefully' test (which asserted the buggy percent:0) with a #3233 no-op
regression (100% record preserved, updated:false), and add a negative-space
guard: plans exist but none done must still write a legitimate 0%.

RED — fails on next; fix follows.

* fix(#3233): no-op state update-progress when the milestone scan finds zero plans

cmdStateUpdateProgress mapped 0/0 through clampPercent to 0% and unconditionally
rewrote the body Progress line, so after /gsd-complete-milestone archived the
phases (.planning/phases/ empty, scope COMPLETE) a routine update-progress run
destroyed the shipped record ([██████████] 100% → [░░░░░░░░░░] 0%).

Add an early-return no-op when totalPlans === 0 — mirroring the established
scope-withholding no-op (stderr WARNING + {updated:false, reason}) and
computeProgressPercent's null-for-empty contract ('nothing to measure' ≠ '0%
done'). The legitimate 0% case (plans exist, none summarized) is unaffected:
totalPlans > 0 reaches clampPercent(0, N>0) = 0 and writes 0% as before.

* test(#3233): unshadow 'Progress field missing' — clear the zero-plans guard

The new totalPlans===0 no-op guard fires before the 'Progress field not found'
branch, so the existing 'returns error when Progress field missing' test (no
phase dirs → 0 plans) was passing for the wrong reason and that branch lost
coverage. Give that test a phase dir + PLAN so totalPlans > 0 clears the guard
and it reaches the branch it is named for. (Isolated review finding.)

* chore(#3233): add changeset fragment

* chore(#3233): backfill changeset PR number (#3375)

---------

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

src/state.cts

@@ -804,6 +804,27 @@ function cmdStateUpdateProgress(cwd: string, raw: boolean): void {
     return;
   }
 
+  // #3233: zero plans in the current-milestone phases means there is nothing to
+  // measure — most often the milestone was just closed and its phases archived
+  // (.planning/phases/ empty, but scope COMPLETE — "a real empty"). clampPercent
+  // maps 0/0 to 0%, which would clobber the shipped Progress record (e.g.
+  // [██████████] 100% → [░░░░░░░░░░] 0%). No-op instead, mirroring the
+  // scope-withhold above and computeProgressPercent's null-for-empty contract
+  // ("nothing to measure" ≠ "0% done"). The legitimate 0% case (plans exist,
+  // none summarized → clampPercent(0, N>0) = 0) is unaffected: totalPlans > 0.
+  if (totalPlans === 0) {
+    process.stderr.write(
+      `[gsd-tools] WARNING: state update-progress skipped — no plans found in current-milestone phases (0 plans). ` +
+      `STATE.md's Progress field was left unchanged (milestone archived?).\n`
+    );
+    output(
+      { updated: false, reason: 'no plans found in current-milestone phases — STATE.md left unchanged (milestone archived?)' },
+      raw,
+      'false',
+    );
+    return;
+  }
+
   const percent = clampPercent(totalSummaries, totalPlans);
   const barWidth = 10;
   const filled = Math.round(percent / 100 * barWidth);