release: stamp v0.51.0 — Kanban v1 launch

CHANGELOG.md: full v0.51.0 entry covering the 12-commit Kanban stack
(#1645, #1646, #1647, #1649, #1654, #1655, #1660, #1675) including
multi-board management, SSE event stream, dispatcher contract enforcement,
CSS-injection fix, archive race fix, mobile responsive, and 35 new
Kanban-specific tests (33 -> 68).

ROADMAP.md, TESTING.md: bumped to v0.51.0 / 4356 tests / 'Kanban v1 launch'.

Major version bump from 0.50.x -> 0.51.0 reflects the size and significance
of the feature: first-party-compatible Kanban surface (CRUD on /api/kanban/boards
+ real-time SSE event stream) parity-verified against the Hermes Agent
dashboard plugin. Independent review APPROVED, Opus advisor SHIP, all
SHOULD-FIX absorbed in-release with regression tests.

ROADMAP.md

@@ -2,7 +2,7 @@
 
 > Web companion to the Hermes Agent CLI. Same workflows, browser-native.
 >
-> Last updated: v0.50.297 (May 04, 2026) — 4288 tests collected
+> Last updated: v0.51.0 (May 04, 2026) — 4356 tests collected — Kanban v1 launch
 > Test source: `pytest tests/ --collect-only -q`
 > Per-version detail: see [CHANGELOG.md](./CHANGELOG.md)