Session handoff for continuing work across sessions.

Stores "what I was doing", "what's next", and "artifacts to continue with" so that when a session resumes, the context is immediately available.

Usage

# At end of session
Recollect.Handoff.create(workspace_id,
  what: "Implementing user auth",
  next: ["Add login controller", "Create session middleware"],
  artifacts: ["lib/auth/user.ex", "lib/auth/token.ex"],
  blockers: ["Waiting on API spec"]
)

# At start of next session
{:ok, handoff} = Recollect.Handoff.get(workspace_id)