feat(web): add approval, workspace, fork, and sub-agent protocols

[QiaolongLi1201]

Context

The DeepSeek Harness-compatible Web shell can now render conversation history, queued turns, live tool trajectories, task-run evidence, settings, command search, attachments, and responsive panels using real Moss state.

Several upstream interaction surfaces cannot be made functional without explicit Moss server contracts. They must not be simulated in browser-only state.

Scope

• Add an abortable interactive approval request/resolve protocol for Web turns.
• Define workspace read/write and change-preview endpoints with the existing Moss safety boundary.
• Define an atomic, versioned session fork/rewind transaction.
• Expose sub-agent run topology, status, cancellation, and branch navigation.
• Decide whether runtime model switching is supported per session and expose only redacted provider metadata.
• Preserve loopback binding, same-origin mutation checks, credential redaction, and honest verification states.

Acceptance criteria

• Every mutation is backed by a Moss runtime contract and covered by behavior tests.
• Refresh/reconnect restores all server-owned state without browser-only reconstruction.
• Approval waits are abortable and cannot execute a tool after cancellation.
• Fork/rewind is atomic and never corrupts the source session.
• Web UI enables controls only when the corresponding capability is advertised.
• User guide and beta API stability notes are updated.