service: context session is unimplemented — a prototype cannot seed a service's sessions

[edwin-zvs]

A service is defined by instruction, harness, model, context session, sandbox, channels, and a routing rule. The context session is the one field with no implementation: ServiceConfig has no such field, and nothing seeds a newly created service session from an existing one.

This is the pillar the whole feature was motivated by. The framing was: a coding harness makes it easy to prototype a use case interactively, but turning that prototype into a service is not straightforward. Ingress, routing, and sandbox all shipped. "Turn prototype session as a context" did not — so today an operator reproduces their prototype by hand as an instruction string and loses everything the prototype accumulated.

Open questions worth settling before implementing:

• What is actually carried forward — the full transcript, a compacted summary, or an explicit slice the operator marks?
• Is it a snapshot at definition time, or a live pointer that re-reads the prototype as it evolves?
• How does it compose with instruction — prepended, replacing, or a separate seed turn?
• Cost: replaying a long prototype into every per-event session is expensive, which argues for compaction or caching.

Propagation class would be next session, alongside instruction and model (see spec 0173).