Define the transport-safe SessionService contract

[ashwin-pc]

Stage 2e protocol foundation

Defines the complete box-local SessionService contract before moving production behavior:

• explicit session context for every session operation; no global-current default
• non-optional lifecycle/session/model/command/tree/extension/viewer surface
• box-scoped filesystem API usable before session creation
• full session-scoped Git and artifact APIs
• artifact bytes cross the boundary, never box-local file paths
• raw state/runtime/message facts contain no host-derived activity timestamps
• host-only WebState DTO remains separate
• typed discriminated service events, shutdown, and extension UI requests
• strict JSON transport validation that rejects undefined/non-finite/function values

No routes or session lifecycle code move in this slice.

Validation: compile-time completeness assertions, JSON round-trip tests, typecheck, full unit/E2E suite, build, diff check.

[ashwin-pc]

Closing this experimental stacked refactor and removing its branch as requested.