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>

container/agent-runner/src/db/connection.ts

@@ -8,6 +8,14 @@
  *   outbound.db — container writes responses + acks here; host opens read-only
  *
  * Each file has exactly one writer, so no cross-process lock contention.
+ *
+ * ⚠ Cross-mount visibility: inbound.db MUST be journal_mode=DELETE (set by
+ * the host when the file is created). WAL's `-shm` is memory-mapped and
+ * VirtioFS does not propagate mmap coherency from host to guest, so a
+ * WAL-mode inbound.db would leave this reader frozen on an early snapshot
+ * and it would silently never see new host messages. See
+ * src/session-manager.ts for the full set of cross-mount invariants and
+ * scripts/sanity-live-poll.ts for the empirical validation.
  */
 import Database from 'better-sqlite3';
 import fs from 'fs';