chore(release): stamp v0.51.18 — 5-PR batch (#1783, #1789, #1790, #1791, #1794)

Constituent PRs:
- #1783 (@Sanjays2402) custom provider + :free/:beta/:thinking suffix fix. Closes #1776.
- #1789 (@Michaelyklam) preserve sidebar scrolling while streaming. Closes #1784.
- #1790 (@Michaelyklam) keep workspace open from preview breadcrumb. Closes #1785.
- #1791 (@Michaelyklam) keep assistant-only stream deltas on current turn. Closes #1787.
- #1794 (@nesquena-hermes, APPROVED by @nesquena) UX bundle: rail tooltip
  cascade fix, +new-conversation has-tooltip--bottom-right variant, context-menu
  hover-bg, rename pre-fill via setSelectionRange.

Tests: 4723 → 4747 collected (+24). 4733 passed, 0 failed in 149s.

Pre-release verification:
- All 5 PRs CI-green individually
- File overlaps (style.css + ui.js between #1789 + #1794) auto-merged cleanly
- node -c clean on all 4 changed JS files
- Browser API sanity 11/11 endpoints
- Pre-stamp re-fetch: all PR heads match local rebases
- Opus advisor: SHIP all 5, 0 MUST-FIX, 1 SHOULD-NOTE on test pattern divergence (acceptable)

Closes #1776, #1784, #1785, #1787.

ROADMAP.md

@@ -2,7 +2,7 @@
 
 > Web companion to the Hermes Agent CLI. Same workflows, browser-native.
 >
-> Last updated: v0.51.17 (May 7, 2026) — 4723 tests collected — 2-PR contributor batch (#1780, #1782)
+> Last updated: v0.51.18 (May 7, 2026) — 4747 tests collected — 5-PR batch (#1783, #1789, #1790, #1791, #1794)
 > Test source: `pytest tests/ --collect-only -q`
 > Per-version detail: see [CHANGELOG.md](./CHANGELOG.md)