feat: add Nexus — persistent conversational awareness space with live memory (#1208)

Implements the Nexus: a dedicated conversational-only interface where Timmy
maintains a persistent session backed by his live memory store. Unlike the
main dashboard chat (which includes tool-approval flow), the Nexus is pure
dialogue with semantic memory context surfaced on every exchange.

Changes:
- src/dashboard/routes/nexus.py — GET/POST/DELETE routes; uses dedicated
  `nexus` session_id so history is isolated from the main dashboard chat
- src/dashboard/templates/nexus.html — two-column layout: chat left,
  memory sidebar + teaching panel right
- src/dashboard/templates/partials/nexus_message.html — chat partial with
  OOB memory-hits swap
- src/dashboard/templates/partials/nexus_facts.html — teaching confirmation
  + facts list partial
- src/dashboard/app.py — import and register nexus_router
- src/dashboard/templates/base.html — NEXUS link in INTEL dropdown
- static/css/mission-control.css — Nexus layout, memory sidebar, teaching
  panel styles (no inline CSS)
- tests/dashboard/test_nexus.py — 9 unit tests, all green
- docs/nexus-spec.md — full scope + acceptance criteria for #1208

Fixes #1208

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/css/mission-control.css

@@ -2664,3 +2664,53 @@
   color: var(--bg-deep);
 }
 .vs-btn-save:hover { opacity: 0.85; }
+
+/* ── Nexus ────────────────────────────────────────────────── */
+.nexus-layout { max-width: 1400px; margin: 0 auto; }
+
+.nexus-header { border-bottom: 1px solid var(--border); padding-bottom: 0.5rem; }
+.nexus-title  { font-size: 1.4rem; font-weight: 700; color: var(--purple); letter-spacing: 0.1em; }
+.nexus-subtitle { font-size: 0.8rem; color: var(--text-dim); margin-top: 0.2rem; }
+
+.nexus-grid {
+  display: grid;
+  grid-template-columns: 1fr 320px;
+  gap: 1rem;
+  align-items: start;
+}
+@media (max-width: 900px) {
+  .nexus-grid { grid-template-columns: 1fr; }
+}
+
+.nexus-chat-panel { height: calc(100vh - 180px); display: flex; flex-direction: column; }
+.nexus-chat-panel .card-body { overflow-y: auto; flex: 1; }
+
+.nexus-empty-state {
+  color: var(--text-dim);
+  font-size: 0.85rem;
+  font-style: italic;
+  padding: 1rem 0;
+  text-align: center;
+}
+
+/* Memory sidebar */
+.nexus-memory-hits  { font-size: 0.78rem; }
+.nexus-memory-label { color: var(--text-dim); font-size: 0.72rem; margin-bottom: 0.4rem; letter-spacing: 0.05em; }
+.nexus-memory-hit   { display: flex; gap: 0.4rem; margin-bottom: 0.35rem; align-items: flex-start; }
+.nexus-memory-type  { color: var(--purple); font-size: 0.68rem; white-space: nowrap; padding-top: 0.1rem; min-width: 60px; }
+.nexus-memory-content { color: var(--text); line-height: 1.4; }
+
+/* Teaching panel */
+.nexus-facts-header  { font-size: 0.7rem; color: var(--text-dim); letter-spacing: 0.08em; margin-bottom: 0.4rem; }
+.nexus-facts-list    { list-style: none; padding: 0; margin: 0; font-size: 0.8rem; }
+.nexus-fact-item     { color: var(--text); border-bottom: 1px solid var(--border); padding: 0.3rem 0; }
+.nexus-fact-empty    { color: var(--text-dim); font-style: italic; }
+.nexus-taught-confirm {
+  font-size: 0.8rem;
+  color: var(--green);
+  background: rgba(0,255,136,0.06);
+  border: 1px solid var(--green);
+  border-radius: 4px;
+  padding: 0.3rem 0.6rem;
+  margin-bottom: 0.5rem;
+}