docs(architecture): chat.agent over Sessions reference

On-ramp doc for future Claude sessions and customer-facing docs.
Captures the state of the chat.agent system after the Session
migration (phases A-E + test infra) so the next session doesn't
have to reconstruct it from git log + code:

- Why the migration (run-scoped streams -> Session primitive).
- Session primitive crash course (externalId idempotency,
  SessionHandle.in / .out, SSE resume, S2 direct writes).
- Chat mapping: one Session per chat, externalId = chatId,
  ChatInputChunk tagged union, session-scoped PATs.
- ASCII flow diagrams for first message / subsequent turns / stop /
  upgrade-required.
- Module layout (SDK / core / webapp), token mint sites (3 of them),
  key invariants, public API surface (unchanged / grown / added).
- Known follow-ups (Phase F deferred: MCP agentChat tool, AgentView
  dashboard component, ai-chat Next.js UI smoke, constant deletion).
- Smoke test sequences + git trail so a future reader can bisect.

Lives under .claude/architecture/ (repo-local notes directory, not
shipped to customers).

Author: ericallam