v1.15.3

## What's New

Engram now supports best-effort prompt context capture directly from `mem_save`. Human-driven memories can retain the user prompt that generated them without requiring every agent protocol to manually pair every observation with a separate prompt save.

## Changes

### Features
- **`mem_save.capture_prompt`**: Added optional `capture_prompt` boolean, defaulting to `true`.
- **Prompt-aware saves**: When the MCP process has current prompt context for the same project/session, `mem_save` records that prompt alongside the observation.
- **Pipeline opt-out**: Automated artifacts such as SDD progress/spec/design saves can pass `capture_prompt=false` to avoid noisy prompt rows.
- **Prompt dedupe**: Reuses existing prompt rows for the same project + session + exact content instead of inserting duplicates.

### Integration Notes
- `mem_save` does not invent prompt content. Plugins or protocols still need to feed prompt context, typically through `mem_save_prompt`, before automatic capture can attach it to later saves.
- `mem_save_prompt` now feeds MCP session activity in-process, so a later `mem_save` can auto-capture the same prompt without duplicating it.

## Upgrade

```bash
brew upgrade engram
```

## Links
- PR: https://github.com/Gentleman-Programming/engram/pull/309
- Issue: https://github.com/Gentleman-Programming/engram/issues/308