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/db/sessions.ts

@@ -93,7 +93,10 @@ export function deletePendingQuestion(questionId: string): void {
 
 // ── Pending Approvals ──
 
-export function createPendingApproval(pa: Partial<PendingApproval> & Pick<PendingApproval, 'approval_id' | 'request_id' | 'action' | 'payload' | 'created_at'>): void {
+export function createPendingApproval(
+  pa: Partial<PendingApproval> &
+    Pick<PendingApproval, 'approval_id' | 'request_id' | 'action' | 'payload' | 'created_at'>,
+): void {
   getDb()
     .prepare(
       `INSERT INTO pending_approvals