fix(newos4): rollback keeps scaffolding, checkpoints cannot collide, tests gate the build

Review of the 08:30 implementer run (v4.6.102, checkpoint name fix). The shipped fix was
correct; these are the defects it introduced or left behind, each measured first.

- virtual-fs.js rollback(): the snapshot strips _snapshots/_sessions/.trash for bloat. Restoring
  it verbatim deleted every checkpoint, every agent session and the trash, made rollback
  irreversible and unrepeatable. Re-attach the live copies after the root swap.
- checkpoint(): names are millisecond-resolution, so tight calls overwrote each other
  (measured 6 calls -> 3 snapshots). Dedupe with a -N suffix, refuse names containing "/"
  (written but unlistable), and prune by snapshot createdAt instead of filename sort, which
  let a custom name ("aaa-oldest") evict newer auto-named checkpoints.
- module-predictor.js: legacy pre-v2 points have no `usable`, so a 0.079 s aborted run
  re-entered similarity as evidence and floored every prediction at 1 s. Backfill `usable`
  from the measurement on load, drop the ones it rejects, add a belt-and-braces guard in
  _findSimilar and count only usable points as actual executions.
- newos4-maintain.py: run every node-runnable modules/test/test-*.js on each maintenance pass.
  The v4.6.102 regression was written but no job ever executed it.
- newos4-steer.py: `done` now requires --commit <sha> that exists in the repo, and gains
  `reopen`. A written plan was being closed as done at 06:17 while the predictor fix shipped
  nowhere, so the implementer's pre-flight queue was empty.
- steering.json: predictor item reopened, checkpoint item stamped with f7249ff.
- maintenance board: MAINT-002/005/015 were fixed in v4.6.81-85 but still listed open/high;
  closed with their commit ids, and MAINT-010/016 annotated with the probes that keep them open.

Tests: test-vfs-checkpoint 23/23 (RED 14/9 before), test-predictor-accuracy 38/38
(T1-T3 RED on HEAD), maintain --fix all PASS, browser round-trip
"Checkpoint created: <name>" -> "Rolled back to checkpoint: <name>" on v4.6.103.

Also carries the already-deployed 2026-10-26 syntax-gate hunk in newos4-maintain.py (syntax_failures + gated rebuild) that had never been committed.

1 files changed, 55 insertions, 0 deletions

diff --git a/modules/test/test-vfs-checkpoint.js b/modules/test/test-vfs-checkpoint.js
index 03a8c8a..65f32dc 100644
--- a/modules/test/test-vfs-checkpoint.js
+++ b/modules/test/test-vfs-checkpoint.js
@@ -84,6 +84,61 @@ function ok(name, cond, detail) {
         'output=' + JSON.stringify(out.join('\n')));
 }
 
+// D. same-millisecond calls must not overwrite each other
+{
+    const fs = mkFs();
+    const names = [];
+    // 4 calls: one more than the cap of 3, so every name is still stored when the next one is
+    // minted (at 6+, prune legitimately frees a name and it may be reused).
+    for (let i = 0; i < 4; i++) names.push(fs.checkpoint(''));
+    ok('D1 four unnamed checkpoints are four distinct names', new Set(names).size === 4,
+        'names=' + JSON.stringify(names));
+    ok('D2 prune still caps the store at three', fs.listCheckpoints().length === 3,
+        'stored=' + JSON.stringify(fs.listCheckpoints()));
+}
+
+// E. prune keeps the NEWEST three, not the alphabetically last
+{
+    const fs = mkFs();
+    fs.checkpoint('aaa-oldest');
+    fs.checkpoint('');
+    fs.checkpoint('');
+    fs.checkpoint('');          // 4th -> prune
+    const left = fs.listCheckpoints();
+    ok('E1 oldest custom-named checkpoint was pruned', !left.includes('aaa-oldest'), 'left=' + JSON.stringify(left));
+    ok('E2 three checkpoints remain', left.length === 3, 'left=' + JSON.stringify(left));
+    ok('E3 rollback still works after pruning', fs.rollback(left[0]) === true);
+}
+
+// F. rollback must not destroy scaffolding
+{
+    const fs = mkFs();
+    fs.mkdirRecursive('/_sessions/agent1');
+    fs.writeFile('/_sessions/agent1/log.json', '{"turns":3}');
+    fs.mkdirRecursive('/.trash');
+    fs.writeFile('/.trash/junk.txt', 'x');
+    fs.writeFile('/work.txt', 'before');
+    const name = fs.checkpoint('keep-me');
+    fs.writeFile('/work.txt', 'after');
+    ok('F1 rollback succeeds', fs.rollback(name) === true);
+    ok('F2 file restored', fs.readFile('/work.txt') === 'before', 'got ' + fs.readFile('/work.txt'));
+    ok('F3 checkpoint survived rollback', fs.listCheckpoints().includes('keep-me'),
+        'list=' + JSON.stringify(fs.listCheckpoints()));
+    ok('F4 sessions survived rollback', !!fs.readDir('/_sessions'));
+    ok('F5 trash survived rollback', !!fs.readDir('/.trash'));
+    ok('F6 rollback is repeatable', fs.rollback('keep-me') === true);
+}
+
+// G. a name with a slash is refused, not silently unlistable
+{
+    const fs = mkFs();
+    let threw = false;
+    try { fs.checkpoint('a/b'); } catch (e) { threw = true; }
+    ok('G1 slash in name throws', threw);
+    ok('G2 nothing was written', fs.listCheckpoints().length === 0,
+        'list=' + JSON.stringify(fs.listCheckpoints()));
+}
+
 if (fails === 0) {
     console.log('test-vfs-checkpoint: ' + passes + ' passed, 0 failed');
 } else {