Adds the session_checkpoints section to the control-plane DB substrate
architecture doc: schema version and row identity, the column groups, the
public API surface, and the hard rules (redaction at write, reconcile
rather than blind restore, unknown live state is not a mismatch, drain
fails closed on an incomplete checkpoint, no transcript storage).
Satisfies acceptance criterion 1 of #660 ("Schema documented and
versioned") for the implementation added in 7e18dcc.
Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
jcwalker3andClaude Opus 4.8
42657b3b65