docs(v2): cross-mount invariants + diagrams; inline a2a routing

- session-manager.ts: shrink the cross-mount invariant header from 31
  lines to 12, keeping each invariant's cause and consequence inline.
- agent-runner/db/connection.ts: parallel cross-mount comment for the
  container-side reader (inbound.db must be journal_mode=DELETE).
- agent-runner/db/messages-out.ts: document that even/odd seq parity
  is load-bearing — seq is the agent-facing message ID returned by
  send_message and consumed by edit_message / add_reaction, looked
  up across both tables.
- v2-checklist.md: record the cross-mount invariants and seq parity
  under Core Architecture so future "simplifications" don't regress
  them.
- scripts/sanity-live-poll.ts: empirical validation harness for the
  three cross-mount invariants — flips each one and observes silent
  message loss / corruption.
- delivery.ts: inline routeAgentMessage at its single callsite (-17
  net lines). The wrapper added more boilerplate than it factored.
- docs/v2-architecture-diagram.{md,html}: rendered Mermaid diagrams
  of the v2 system, message flow, named destinations, entity model,
  and the two-DB split.
- channels/adapter.ts, chat-sdk-bridge.ts, credentials.ts,
  db/sessions.ts, db/db-v2.test.ts: prettier format pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/channels/adapter.ts

@@ -29,7 +29,9 @@ export interface ChannelSetup {
   onAction(questionId: string, selectedOption: string, userId: string): void;
 
   /** Credential collection hooks — used by chat-sdk-bridge to route the modal flow. */
-  getCredentialForModal?(credentialId: string): { name: string; description: string | null; hostPattern: string } | null;
+  getCredentialForModal?(
+    credentialId: string,
+  ): { name: string; description: string | null; hostPattern: string } | null;
   onCredentialReject?(credentialId: string): void;
   onCredentialSubmit?(credentialId: string, value: string): void;
   onCredentialChannelUnsupported?(credentialId: string): void;

src/channels/chat-sdk-bridge.ts

@@ -188,10 +188,7 @@ export function createChatSdkBridge(config: ChatSdkBridgeConfig): ChannelAdapter
             }
             try {
               const modalChildren = [
-                CardText(
-                  pending.description ??
-                    `Enter the value for ${pending.name} (host: ${pending.hostPattern}).`,
-                ),
+                CardText(pending.description ?? `Enter the value for ${pending.name} (host: ${pending.hostPattern}).`),
                 TextInput({
                   id: 'value',
                   label: pending.name,