v1.13.0: message_parts table + dual-write at every tool_calls/tool_results site

Adds a granular message_parts table (one row per text/tool_call/tool_result
chunk) without changing any read path. Old messages.content / tool_calls /
tool_results columns remain authoritative for v1.13.0; this dispatch is
write-only mirroring so the AI SDK migration in v1.13.1 can flip read
authority without a backfill window.

Schema:
  CREATE TABLE message_parts (id, message_id FK ON DELETE CASCADE,
    sequence int, kind text CHECK (text|tool_call|tool_result|reasoning|step_start),
    payload jsonb, created_at, UNIQUE (message_id, sequence))

New module services/inference/parts.ts with two pure derive helpers
(partsFromAssistantMessage, partsFromToolMessage) and insertParts that
fan-outs a multi-row INSERT via postgres-js.

Wired dual-write at every site that writes tool_calls or tool_results:
- tool-phase.ts: assistant finalize UPDATE, executed-tool UPDATE,
  ask_user_input sentinel UPDATE
- messages.ts answer flow: DELETE pending tool_result part + INSERT
  answered one inside the existing sql.begin
- skills.ts: synthetic assistant + tool INSERTs both inside existing tx
- chats.ts fork: CTE clones parts via ROW_NUMBER pairing (source→dest
  message id mapping in one statement, no N+1)
- error-handler.ts finalizeCompletion: text part for plain text-only
  assistant turns

Deviation: tool-phase.ts finalize UPDATEs and finalizeCompletion text-part
write are not wrapped in fresh sql.begin transactions. Safe in v1.13.0
because JSON columns are authoritative for reads. v1.13.1 must wrap these
sites before flipping read authority — TODO comments added at each
unwrapped site referencing v1.13.1.

Tests: 8 new unit tests for the derive helpers in
services/__tests__/parts.test.ts. Existing 162 tests untouched. 170 total.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/server/src/schema.sql

@@ -32,6 +32,23 @@ CREATE TABLE IF NOT EXISTS messages (
 
 CREATE INDEX IF NOT EXISTS idx_messages_session ON messages(session_id, created_at);
 
+-- v1.13.0: granular message parts table for AI SDK migration. Old
+-- messages.content / tool_calls / tool_results columns stay authoritative
+-- for reads in v1.13.0; this table is dual-written so the swap can happen
+-- in a later dispatch without a backfill window. ON DELETE CASCADE means
+-- removing a message removes its parts in one go.
+CREATE TABLE IF NOT EXISTS message_parts (
+  id uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  message_id uuid NOT NULL REFERENCES messages(id) ON DELETE CASCADE,
+  sequence int NOT NULL,
+  kind text NOT NULL,
+  payload jsonb NOT NULL,
+  created_at timestamptz NOT NULL DEFAULT clock_timestamp(),
+  CONSTRAINT message_parts_kind_chk CHECK (kind IN ('text', 'tool_call', 'tool_result', 'reasoning', 'step_start')),
+  CONSTRAINT message_parts_seq_uniq UNIQUE (message_id, sequence)
+);
+CREATE INDEX IF NOT EXISTS message_parts_msg_seq_idx ON message_parts (message_id, sequence);
+
 ALTER TABLE messages ADD COLUMN IF NOT EXISTS tokens_used INTEGER;
 ALTER TABLE messages ADD COLUMN IF NOT EXISTS ctx_used INTEGER;
 ALTER TABLE messages ADD COLUMN IF NOT EXISTS ctx_max INTEGER;