Add chunked embeddings with overlap and chunk stitching

[aurexav]

Context
Single-vector notes dilute important content in long memories. QMD uses fixed-size chunks with overlap to improve recall at boundaries.

Goal
Improve recall for long memories by indexing overlapping chunks and merging results back to the note level.

Scope

• Add configurable chunk size and overlap.
• Store chunk embeddings tied to note_id, sequence, and offsets.
• Retrieve by chunk similarity and stitch adjacent chunks into a single memory result.

Acceptance Criteria

• Long-memory recall improves without degrading short-note precision.

References

• https://github.com/tobi/qmd

[aurexav]

Closed by commits add223a and ef40a58.