[P3][AGENT-07] Cross-agent delegation and independent verification

Programme: #1
Epic: #20

## Objective
Allow multiple agents/providers to cooperate on one durable task while preserving scoped authority, isolated mutable state and verification independent of provider agreement.

## Scope
- Delegate specialist jobs with bounded context/tool/output schemas.
- Separate repository worktrees and artifact namespaces for parallel code agents.
- Provider switching mid-task using shared GoalContract, evidence and artifact state.
- Critique/review workers for subjective outputs without treating model consensus as proof.
- Deterministic verifiers: tests, type checks, hashes, connector receipts, account state and exact constraints.
- Staged merge/reconciliation of competing patches/results.
- Concurrency/resource locking for shared accounts/budgets/desktops.

## Product rules
- Agents cannot delegate additional authority beyond their job grants.
- Changing model/provider must not discard user constraints/evidence/workspace state.
- Two models agreeing is not sufficient verification for externally checkable facts/actions.

## Acceptance criteria
- [ ] Provider switch preserves task constraints, source evidence and artifacts.
- [ ] Parallel coding agents use separate worktrees and cannot silently overwrite each other.
- [ ] Conflicting outputs are surfaced/reconciled rather than first-writer-wins.
- [ ] Unverified/disagreeing results cannot mark the task Completed.
- [ ] Deterministic checks are preferred and recorded when available.
- [ ] Cancellation propagates to all active delegated jobs and invalidates leases.

## Dependencies
- AGENT-01
- AGENT-02
- CODE-01

**First phase:** P3
**Maturity target:** P5
**Owner:** harness-pc-providers

No files, tests, or entry points are named. Start by reviewing dependencies AGENT-01, AGENT-02, and CODE-01, then map the first-phase scope against the acceptance criteria. Done means delegation, provider switching, reconciliation, verification, cancellation, and resource isolation satisfy the listed product rules.

Written by the indexing model from the issue text.