[P3][PC-04] Controlled unattended desktop sessions

Programme: #1
Epic: #23

## Objective
Support unattended/native application work in isolated desktop sessions or VMs without sharing the user's live physical input stream or broad host credentials.

## Scope
- Dedicated local/VM desktop session lifecycle: create, boot/login where authorized, execute, suspend, snapshot, recover, destroy.
- Scoped file mounts and credential handles; no ambient host home/profile access.
- Separate virtual display/input channel per session.
- Session recording/screenshot retention controls and privacy metadata.
- Network/egress policy per session.
- Crash/reboot recovery and task reconciliation.
- Human inspection/takeover path that clearly indicates this is not the live desktop.

## Isolation rules
- OS virtual desktops alone do not count as independent security/input boundaries.
- Parallel unattended jobs require genuinely isolated sessions.
- Session secrets/state must not become visible to other sessions/accounts.

## Acceptance criteria
- [ ] Two parallel sessions can execute without sharing one keyboard/mouse stream.
- [ ] Each session sees only its scoped files, credentials and destinations.
- [ ] Session crash/restart preserves task state without blindly repeating external writes.
- [ ] Captured visual data follows explicit retention/sensitivity policy.
- [ ] Leaked state from one fixture session cannot access another account/session.
- [ ] UI clearly distinguishes live co-pilot control from isolated unattended execution.

## Dependencies
- PC-01
- SEC-04

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

Start by reviewing the linked dependencies PC-01 and SEC-04, then map the objective and isolation rules to the acceptance criteria. Done means satisfying all listed criteria, including independent parallel sessions, scoped access, safe crash recovery, controlled visual-data retention, isolation between sessions, and clear UI distinctions.

Written by the indexing model from the issue text.