## Problem
Users hit this error when agent tool results contain Unicode null
characters:
```
persist step: insert tool result: pq: unsupported Unicode escape sequence
```
PostgreSQL's `jsonb` type rejects `\u0000` (Unicode null, U+0000) with
that error, even though it's valid JSON per RFC 8259. Tool results from
agents can contain this sequence — e.g. binary data, C-style strings, or
certain API responses.
## Root cause
`MarshalToolResult` and `MarshalContent` in `chatprompt.go` serialize
content blocks to JSON and pass them directly to `InsertChatMessage`
which casts to `::jsonb`. Go's `json.Marshal` / `json.Valid` accept
`\u0000`, but Postgres does not.
## Fix
Added `sanitizeJSONForPG()` which strips `\u0000` escape sequences from
serialized JSON before insertion. Uses `bytes.Contains` as a fast-path
check to avoid allocation when no null bytes are present (the common
case).
Applied to both `MarshalContent` (assistant messages) and
`MarshalToolResult` (tool result messages).
Kyle Carberry