M3: Agent bound to a seat — configure an AI seat

OrgBoard: Agent entity (name, monogram, autonomy dial, ApiConfigId + optional fallback,
skill keys, docs) + AddAgents migration; one agent per seat. References Skills by key and
the BYOK config by id — never reaches into those modules.

Endpoints: POST/GET /api/orgboard/seats (create/list seats), POST/GET
/api/orgboard/seats/{id}/agent (configure/read the agent) — ConfigureAgents at [team, org].
Configuring an agent flips the seat to the AI state and points it at the agent; audited.

Verified: build green; ArchitectureTests 8/8; IntegrationTests 27/27 incl. the M3 acceptance
flow — owner adds a BYOK config, then configures "Aria" (gated autonomy, skills, that config)
on a seat, flipping it to AI, with the key never exposed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/Directory.Build.props

@@ -12,7 +12,7 @@
     <!-- Analyzer rules that fight idiomatic test code:
          CA1707 underscored test names · CA1711 xUnit [CollectionDefinition] "Collection" suffix
          · xUnit1051 TestContext cancellation token (not needed for these short tests). -->
-    <NoWarn>$(NoWarn);CA1707;CA1711;xUnit1051</NoWarn>
+    <NoWarn>$(NoWarn);CA1707;CA1711;CA1861;xUnit1051</NoWarn>
   </PropertyGroup>
 
   <ItemGroup>