Add horizontal slice metadata for wide tasks

[nathanwhit]

Summary

• Add slice IDs and scope metadata to worker and spawn plans, work-plan items, execution nodes, graph nodes, and UI types.
• Inject slice assignments into worker prompts so horizontally partitioned workers stay inside their owned scope and report cross-slice dependencies.
• Teach the replanning prompt/schema about wide task decomposition and preserve slice metadata through projections and compact prompt state.

Validation

• GOCACHE=/tmp/aged-go-build go test ./internal/orchestrator ./internal/eventstore ./internal/httpapi

[nathanwhit]

Reviewed the horizontal slice metadata change. The metadata is carried through plan decoding, execution nodes, graph projection, worker results, retry state, prompts, and web types, with regression coverage for initial graph propagation. No blockers found; CI is green. I am leaving this as a review comment rather than an approval because this PR is authored by the active GitHub account.