feat(webui): Runtime and session view (Phase 1) (Closes #641)

Compose runtime health with inventory sessions/namespaces/worktrees into a
live /sessions page and JSON API. Surface stale PID/lease flags and durable
contamination markers when detectable. Recovery links name sanctioned
reconnect/restart paths only — no kill controls.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

webui/nav.py

@@ -48,7 +48,7 @@ NAV_GROUPS: tuple[NavGroup, ...] = (
     )),
     NavGroup("Runtime/Sessions", (
         NavItem("/runtime", "Runtime health"),
-        NavItem("/sessions", "Sessions", "stub"),
+        NavItem("/sessions", "Sessions"),
     )),
     NavGroup("Projects", (
         NavItem("/projects", "Projects"),
@@ -76,11 +76,6 @@ NAV_GROUPS: tuple[NavGroup, ...] = (
 # issues of epic #631. Each maps a path to (title, description). Routes are
 # registered so nav links resolve to a graceful, read-only stub page.
 STUB_PAGES: dict[str, tuple[str, str]] = {
-    "/sessions": (
-        "Sessions",
-        "Active session, capability, and role inventory. Backed by the unified "
-        "inventory API (#636) once it lands.",
-    ),
     "/inventory": (
         "Inventory",
         "Unified sessions, leases, locks, namespaces, and worktree inventory. "