EPIC: Harness and required provider integrations

Programme: #1

Own durable task graphs, scoped orchestration, the common AgentSession contract, built-in local agent integration and required Codex / Claude Code / Cursor / Grok adapters.

## Child issues
- [ ] #54 AGENT-01 — Task graph and scoped orchestration
- [ ] #55 AGENT-02 — Common AgentSession adapter interface
- [ ] #56 AGENT-03 — Codex integration
- [ ] #57 AGENT-04 — Claude Code integration
- [ ] #58 AGENT-05 — Cursor integration
- [ ] #59 AGENT-06 — Grok integration
- [ ] #60 AGENT-07 — Cross-agent delegation and independent verification

## Cross-cutting gates
Providers never own durable product state; subagents cannot expand authority; cancellation/leases/checkpoints are enforced; provider-specific differences remain explicit; external CLIs are confined before project hooks/tools can execute.

This is an umbrella epic covering child issues #54–#60, from task graphs and the AgentSession interface through provider adapters and cross-agent delegation. Start by reading the linked child issues and the cross-cutting gates in this issue. Done means the child work is completed while preserving durable state ownership, authority limits, cancellation, leases, checkpoints, and provider isolation.

Written by the indexing model from the issue text.