v1.15.4

## What's New

This patch hardens the `mem_save` prompt capture release after Judgment Day review. It fixes session activity tracking, clarifies prompt-capture documentation, and adds regression coverage around the failure paths.

## Changes

### Bug Fixes
- **Correct session activity**: `mem_save` now records save activity against the resolved `session_id`, including explicit caller-provided sessions.
- **Nil-safe activity tracking**: reusable handler paths tolerate missing `SessionActivity` without panics.
- **Non-fatal prompt capture**: prompt capture failures remain best-effort and do not fail the observation save.

### Tests
- Added regressions for explicit `session_id` activity tracking.
- Added nil-activity safety coverage.
- Added non-fatal prompt capture failure coverage.
- Added prompt dedupe sync-mutation assertion.

### Documentation
- Clarified that only `mem_save` and `mem_save_prompt` accept the narrow ambiguous-project recovery override.
- Reworded “no auto-capture” docs to “no raw tool-call firehose,” while documenting best-effort prompt capture tied to curated `mem_save` calls.
- Documented that prompt capture is process-local: the same MCP process lifecycle must receive prompt context before a later `mem_save` can attach it.

## Upgrade

```bash
brew upgrade engram
```

## Links
- PR: https://github.com/Gentleman-Programming/engram/pull/311
- Issue: https://github.com/Gentleman-Programming/engram/issues/310