feat: incremental capture, every-message recall, metadata stripping

Ported from OpenClaw supermemory-auto best practices:

1. Incremental capture (crash-safe):
   - Save each user+assistant exchange immediately after assistant responds
   - No more waiting for session end — if OpenCode crashes, memories are safe
   - Uses PERSONAL_ENTITY_CONTEXT for extraction guidance
   - Configurable: incrementalCapture (default: true), maxCaptureChars (5000)

2. Every-message recall:
   - Changed reinjectEveryN default from 0 to 1
   - Context is now re-injected on EVERY message, not just the first
   - Ensures topic changes mid-session get fresh relevant context

3. Metadata stripping:
   - Strip supermemory-context blocks from queries before recall
   - Strip JSON metadata blocks and untrusted sender info
   - Cleaner queries = better recall accuracy
   - Query trimmed to 300 chars max