Merge remote-tracking branch 'origin/master' into session-surface

Reconcile the session-surface feature with master's package reorg and
simplifications:

- Adopt master's folded usage (assistant/message.usage; standalone `usage`
  event dropped) and re-attach surface metadata (surfaceOp/sourceEventSeqs).
- Add surface opts to master's new max-tokens assistant/message append.
- Port surface columns onto the coordinator-refactored SQLite backend at its
  new path; drop the dead v1->v2 migration (bump-and-reject, no migration per
  pre-release policy).
- Move the session-surface RFC into implemented/architecture/ and refresh its
  stale body (no migration, SESSION_FORMAT_VERSION=0, renamed package paths).
- Update the core-data-structures catalog SessionEvent blocks for the two new
  surface fields; regenerate the cordis catalog.
- Re-harvest ACP snapshot fixtures (keyless replay) to carry surface metadata.

examples/acp-agent/tests/snapshots/error-finish/input.json

@@ -0,0 +1,7 @@
+{
+  "steps": [
+    { "op": "initialize" },
+    { "op": "newSession" },
+    { "op": "promptExpectError", "text": "This prompt triggers a recorded provider error." }
+  ]
+}

examples/acp-agent/tests/snapshots/error-finish/replay.override.json

@@ -0,0 +1,3 @@
+[
+  { "kind": "throw", "chunks": [], "message": "simulated provider error (HTTP 401)", "code": "AUTH", "status": 401 }
+]

examples/acp-agent/tests/snapshots/error-finish/session.jsonl

@@ -0,0 +1,6 @@
+{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"turn/start","seq":0,"time":0,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
+{"type":"user/message","seq":1,"time":0,"data":{"content":[{"type":"text","text":"This prompt triggers a recorded provider error."}],"source":{"kind":"user"}},"surfaceOp":"append"}
+{"type":"step/start","seq":2,"time":0,"data":{"turn":1,"step":1}}
+{"type":"step/end","seq":3,"time":0,"data":{"turn":1,"step":1}}
+{"type":"turn/end","seq":4,"time":0,"data":{"turn":1,"reason":{"kind":"error","step":1,"message":"simulated provider error (HTTP 401)","code":"AUTH"}}}

examples/acp-agent/tests/snapshots/error-finish/stdout.golden.jsonl

@@ -0,0 +1,3 @@
+{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":1,"agentInfo":{"name":"deepseek-harness-acp","version":"0.0.1"},"agentCapabilities":{"loadSession":true,"promptCapabilities":{"image":false,"audio":false,"embeddedContext":false}},"authMethods":[]}}
+{"jsonrpc":"2.0","id":2,"result":{"sessionId":"{{sessionId}}"}}
+{"jsonrpc":"2.0","id":3,"error":{"code":-32603,"message":"Internal error: turn failed: simulated provider error (HTTP 401)"}}