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/routes/skills.ts

@@ -90,11 +90,26 @@ export function registerSkillsRoutes(
           VALUES (${sessionId}, ${chat.id}, 'assistant', '', ${sql.json(toolCalls as never)}, 'complete', clock_timestamp())
           RETURNING id
         `;
+        // v1.13.0: dual-write the synthetic assistant message's tool_call.
+        // Single skill_use tool_call, no text content, so one part at seq 0.
+        await tx`
+          INSERT INTO message_parts (message_id, sequence, kind, payload)
+          VALUES (${synthAssistant!.id}, 0, 'tool_call', ${tx.json({
+            id: toolCallId,
+            name: 'skill_use',
+            args: { name: skill_name },
+          } as never)})
+        `;
         const [toolMsg] = await tx<{ id: string }[]>`
           INSERT INTO messages (session_id, chat_id, role, content, tool_results, status, created_at)
           VALUES (${sessionId}, ${chat.id}, 'tool', '', ${sql.json(toolResults as never)}, 'complete', clock_timestamp())
           RETURNING id
         `;
+        // v1.13.0: dual-write the synthetic tool result (the skill body).
+        await tx`
+          INSERT INTO message_parts (message_id, sequence, kind, payload)
+          VALUES (${toolMsg!.id}, 0, 'tool_result', ${tx.json(toolResults as never)})
+        `;
         const [userMsg] = await tx<{ id: string }[]>`
           INSERT INTO messages (session_id, chat_id, role, content, status, created_at)
           VALUES (${sessionId}, ${chat.id}, 'user', ${userText}, 'complete', clock_timestamp())