Support many-to-many note–chat linking

[devin-ai-integration]

Per docs/adr/0001-json-chats-and-markdown-notes.md (line ~83), linked_chats is modeled as a list but the MVP only uses a single continuation-chat slot per note.

Problem

• There is a schema/behavior gap: the schema allows many links, but behavior supports only one continuation chat per note.

Suggested direction

• Support true many-to-many note–chat linking to close the gap.

References

• docs/adr/0001-json-chats-and-markdown-notes.md (line ~83)