chore(release): stamp v0.51.16 — 3-PR batch (#1768, #1778, #1779)

Constituent PRs:
- #1768 (@franksong2702) serialize Anthropic env fallback reads. Closes #1736.
- #1778 (@Michaelyklam) preserve CLI session tool metadata. Closes #1772.
- #1779 (@Michaelyklam) reset model picker on session switch. Closes #1771.
  AUTO-FIX: Opus stage-310 caught a regression in the new !hasSessionModel
  branch — it dropped the deferModelCorrection guard that the parallel
  else-branch keeps. Fired spurious /api/session/update POSTs against
  imported/read-only CLI sessions whose model field reads 'unknown' (the
  exact surface #1778 introduces in this same release). Wrapped the new
  branch's _persistSessionModelCorrection call + state mutation in
  if(!deferModelCorrection). Added test_sync_topbar_does_not_persist_correction_while_model_resolution_deferred
  regression test covering both empty and 'unknown' fast-path interaction.

Tests: 4694 → 4702 collected (+8). 4695 passed, 4 skipped, 3 xpassed,
0 failed in 141.29s.

Pre-release verification:
- All 3 PRs CI-green individually.
- node -c clean on static/ui.js.
- 11/11 browser API endpoints PASS.
- Pre-stamp re-fetch: all PR heads match local rebases.
- Opus advisor: SHIP #1768 + #1778, #1779 SHOULD-FIX before merge — auto-fix
  applied at stage with regression test, re-verified clean.

Closes #1736, #1771, #1772.

ROADMAP.md

@@ -2,7 +2,7 @@
 
 > Web companion to the Hermes Agent CLI. Same workflows, browser-native.
 >
-> Last updated: v0.51.15 (May 7, 2026) — 4694 tests collected — 4-PR contributor batch + 1 self-built (#1762, #1767, #1769, #1770)
+> Last updated: v0.51.16 (May 7, 2026) — 4702 tests collected — 3-PR contributor batch (#1768, #1778, #1779)
 > Test source: `pytest tests/ --collect-only -q`
 > Per-version detail: see [CHANGELOG.md](./CHANGELOG.md)