Write a handoff row from a PreCompact or SessionEnd hook

T858. Same transport as the session-start hook. The server parses the config, honours enabled and capture_scope, assembles the handoff body from the supplied git facts, and upserts it on external_id through the same bulk-upsert path agent_handoff_save uses — so a re-run for the same Claude Code session UPDATES its row rather than forking a second one. Always answers 204; the hook prints nothing.

T837 adds the optional synthesis_* fields. They carry a summary the CLIENT already finished, on its own machine, from its own transcript: the raw transcript never reaches this endpoint and the server runs no model on this path. With at least one decision the row is written with metadata.wrap_kind = "synthesis" and its description is the Decisions: line boot serves as the tldr; with none it is the mechanical safety net, marked synthesis_status: "degraded" with the client's reason. A degraded snapshot never overwrites a synthesis or seed row already stored under the same external_id.