Add ce_sessions schema (onSchemaInit), CeSessionStore (via
ctx.taskStore.getDatabase()), and CeOrchestrator driving a stage's skill on the
U4 interactive seam: streams thinking/text, persists questions as
awaiting_input, writes the stage artifact on complete, and auto-saves +
emits an observable event on interrupt/error (never silent loss). Lifecycle:
launching/active/awaiting_input/completed/error/interrupted with resume and an
interval-relative staleness rubric. Start/answer/resume/get-state routes.
Streaming transport is client polling of the get-session-state route for v1:
plugin routes have no native SSE and the loader emitEvent is a logging stub, so
true server push needs a host publish-to-/api/events seam (tracked follow-up).
Skill discovery is cwd+prompt-based; forwarding install paths through the seam
is a tracked follow-up.
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