refactor(core): migrate MessageV2.Format to Effect Schema (#23744)

packages/opencode/specs/effect/schema.md

@@ -186,6 +186,80 @@ schema module with a clear domain.
 Major cluster. Message + event types flow through the SSE API and every SDK
 output, so byte-identical SDK surface is critical.
 
+Suggested order for this cluster, starting from the leaves that `session.ts`
+and the SSE/event surface depend on:
+
+1. `src/session/schema.ts` ✅ already migrated
+2. `src/provider/schema.ts` if `message-v2.ts` still relies on zod-first IDs
+3. `src/lsp/*` schema leaves needed by `LSP.Range`
+4. `src/snapshot/*` leaves used by `Snapshot.FileDiff`
+5. `src/session/message-v2.ts`
+6. `src/session/message.ts`
+7. `src/session/prompt.ts`
+8. `src/session/revert.ts`
+9. `src/session/summary.ts`
+10. `src/session/status.ts`
+11. `src/session/todo.ts`
+12. `src/session/session.ts`
+13. `src/session/compaction.ts`
+
+Dependency sketch:
+
+```text
+session.ts
+|- project/schema.ts
+|- control-plane/schema.ts
+|- permission/schema.ts
+|- snapshot/*
+|- message-v2.ts
+|  |- provider/schema.ts
+|  |- lsp/*
+|  |- snapshot/*
+|  |- sync/index.ts
+|  `- bus/bus-event.ts
+|- sync/index.ts
+|- bus/bus-event.ts
+`- util/update-schema.ts
+```
+
+Working rule for this cluster:
+
+- migrate reusable leaf schemas and nested payload objects first
+- migrate aggregate DTOs like `Session.Info` after their nested pieces exist as
+  named Schema values
+- leave zod-only event/update helpers in place temporarily when converting
+  them would force unrelated churn across sync/bus boundaries
+
+`message-v2.ts` first-pass outline:
+
+1. Schema-backed imports already available
+   - `SessionID`, `MessageID`, `PartID`
+   - `ProviderID`, `ModelID`
+2. Local leaf objects to extract and migrate first
+   - output format payloads
+   - common part bases like `PartBase`
+   - timestamp/range helper objects like `time.start/end`
+   - file/source helper objects
+   - token/cost/model helper objects
+3. Part variants built from those leaves
+   - `SnapshotPart`, `PatchPart`, `TextPart`, `ReasoningPart`
+   - `FilePart`, `AgentPart`, `CompactionPart`, `SubtaskPart`
+   - retry/step/tool related parts
+4. Higher-level unions and DTOs
+   - `FilePartSource`
+   - part unions
+   - message unions and assistant/user payloads
+5. Errors and event payloads last
+   - `NamedError.create(...)` shapes can stay temporarily if converting them to
+     `Schema.TaggedErrorClass` would force unrelated churn
+   - `SyncEvent.define(...)` and `BusEvent.define(...)` payloads can keep using
+     derived `.zod` until the sync/bus layers are migrated
+
+Possible later tightening after the Schema-first migration is stable:
+
+- promote repeated opaque strings and timestamp numbers into branded/newtype
+  leaf schemas where that adds domain value without changing the wire format
+
 - [ ] `src/session/compaction.ts`
 - [ ] `src/session/message-v2.ts`
 - [ ] `src/session/message.ts`