devenv image: define the kit-declared volume model for .devenv state #512

Context

Split out from #418. The image README already says .devenv/ lives in the workspace and persists with it, and that recreating a sandbox under --clone discards .devenv/state (including service data such as a Postgres database). It doesn't yet say how a kit should keep that state across recreates.

Scope

1. Decide how a kit declares a volume for .devenv/ (or just .devenv/state) under --clone on both msb and sbx, and whether the eval cache and the process-manager state should be split.
2. Document the model in the image README, with an example kit volumes: entry.
3. Note that devenv 2.2 fixed the SQLite eval cache on virtiofs, which applies to msb/sbx bind mounts. On older devenv, a virtiofs-backed .devenv may hit that bug.

Depends on

Step 3, and any recommendation to put the eval cache on a virtiofs mount, depend on the image shipping devenv 2.2+ (the channel decision in #418).

Verification

• With the documented volume, devenv up -d state (for example a Postgres data dir) survives a sandbox recreate under --clone on msb and on sbx.