Add Session Note Internal

The agent records a "note to self" — an unstructured standing reminder that changes how it behaves for the rest of the conversation (pace/verbosity the user signalled, a constraint, a name correction). Append-only: the agent owns the active set in memory and curates it via supersedes; this route persists each note durably (provenance + the corpus for "what feedback are we getting") and rehydrates the agent on reconnect via the GET below.

Superseding or hitting the agent's soft cap flips active to false — we never delete, so the row survives as analysis material and a future hook for promoting durable notes to cross-conversation memory (hence person_id).