[P0][AGENT-05] Cursor integration

Programme: #1
Epic: #20

## Objective
Integrate Cursor's Agent Client Protocol path behind `AgentSession`, preserving mode/capability differences and strict workspace confinement.

## Scope
- ACP transport and lifecycle integration over supported stdio/session surface.
- Capability/mode negotiation and provider-version pinning.
- Normalize progress, permission requests, artifacts, diagnostics and cancellation.
- Translate runtime-approved resources into a staged workspace rather than ambient host access.
- Surface actual model/account/inference location where the provider exposes it.
- Golden protocol fixtures for supported versions and optional capabilities.

## Security / product rules
- ACP transport does not imply permission to expose all local files/tools.
- Unsupported optional ACP capabilities remain explicitly unsupported.
- Provider account/model identity must be visible for tasks that may transmit private code/context.

## Acceptance criteria
- [ ] Shared bounded coding fixture runs through ACP without terminal scraping.
- [ ] Session cannot access files outside the approved staged workspace.
- [ ] Permission requests normalize into trusted runtime proposals.
- [ ] Cancellation/termination is reflected in task state and invalidates execution leases.
- [ ] Model/account/inference location is disclosed where determinable.
- [ ] Version/capability drift is caught by startup/golden fixtures.

## Dependencies
- AGENT-02

**First phase:** P0
**Maturity target:** P3
**Owner:** harness-pc-providers

Start by reading the AGENT-02 dependency and locating the AgentSession integration surface, then review the ACP transport, lifecycle, capability, and mode requirements in this issue. Use the shared bounded coding fixture and golden protocol fixtures to verify workspace confinement, normalized permissions, cancellation, disclosure, and version or capability drift handling.

Written by the indexing model from the issue text.