Add memory_compactions tool for browsing compaction checkpoints

Queries compaction-type parts in the DB to find session compaction
events, then retrieves the summary text from the adjacent assistant
message. Presents compactions as navigable checkpoints with list and
read modes.

docs/architecture.md

@@ -82,6 +82,7 @@ All backed by read-only `bun:sqlite` queries to `${XDG_DATA_HOME:-$HOME/.local/s
 
 | Tool | Purpose |
 |------|---------|
+| `memory_compactions` | List/read compaction checkpoints for a session |
 | `memory_summary` | Quick counts: projects, sessions, messages, todos |
 | `memory_sessions` | List recent sessions with metadata, sorted by update time |
 | `memory_messages` | Read messages from a specific session as markdown |