feat(execution-core): add execution_incomplete terminal state

[dawidaksamski]

What

A graph could route itself into nothing and report success. When a decision returns a
nextPort no edge carries — deleted edge, renamed branch, never wired — every outgoing
edge is pruned and the run closed as completed with part of the graph never run.

runGraph now records each dead end as { nodeId, port } and, if any were found, emits a
terminal execution_incomplete event in place of execution_completed and writes the
'incomplete' status. Not a failure — nothing threw, so Temporal still closes the Workflow
Execution as Completed. Failure keeps precedence, and the cycle check stays a failure with
its own "stalled" wording.

Dead ends don't stop the run: they're collected run-scoped and reported once at the end, so
parallel branches still deliver their work.

⚠️ Behaviour change

An 'errorRoute' failure with no 'errorRoute' edge now ends the run incomplete — it was
previously documented as a usable silent DLQ. The general rule covers it (the policy names a
port, nothing was wired to it), and excluding it would mean carving out
RESERVED_ERROR_HANDLE. Deliberate absorption is what 'continue' is for; the README now
says so. The test asserting the old behaviour was rewritten, not deleted.

Worth a look in review

Six separate places enumerate terminal states — worker database.ts, backend
drain-events.ts and routes/executions.ts, ai-studio stream adapter, store, and controls.
All six change here; missing one hangs an SSE stream, leaves finished_at null, or strands
the UI mid-run. run-workflow.ts is a comment-only change: its guard is === 'failed', so
the new state correctly falls through.

Naming (incomplete over stalled), why the cycle check stays separate, and the
errorRoute reversal are written up in
packages/execution-core/terminal-states.decision-log.md.