Add support for swapping underlying session sharing connection in terminal view and friends.

[zachbai]

Description

Adds support for swapping out the viewer-side shared session piping for an existing TerminalView/Manager to support continuing cloud agent conversations across VM lifecycle boundary.

Adds SessionSharingInitialLoadMode for ignoring replay events when joining a shared session for a cloud agent task with agent scrollback.

[zachbai]

• Use new cloud mode setup UI for local -> cloud handoff UI #9653
• implement local cloud handoff #9455
• Ensure cloud conversation status updates in response to execution completion #9315
• Refactor AmbientAgentViewModel to handle follow-up run executions. #9316
• Add feature flag, API binding scaffolding for cloud->cloud handoff. #9313
• Add support for swapping underlying session sharing connection in terminal view and friends. #9311 👈 (View in Graphite)
• Remove NotAmbientAgent state from AmbientAgentViewModel. #9310
• master

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@zachbai

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[oz-for-oss]

Overview

This PR reworks the shared-session viewer lifecycle so an ambient agent terminal can attach a fresh follow-up session, adds append-mode scrollback loading, and routes outbound viewer updates through a replaceable current network.

Concerns

• Follow-up append mode still processes replayed agent conversation events after loading scrollback, which can duplicate existing cloud-agent history when the new session replays prior transcript state.
• Ambient session-end handling guards only current_network clearing by session ID; stale events from a closed prior network can still unregister the active share and clear the new write-to-PTY channel.

Verdict

Found: 0 critical, 2 important, 0 suggestions

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[oz-for-oss]

⚠️ [IMPORTANT] Append mode only changes scrollback loading; replay events from the follow-up session are still processed from event 0, so a session that replays the existing agent transcript can duplicate AI history. Preserve the load mode on EventLoop and skip replay events for follow-up joins.

[oz-for-oss]

⚠️ [IMPORTANT] A late SessionEnded from the closed previous network still runs this teardown after a follow-up connects, which unregisters the active share and clears the new write-to-PTY channel. Guard all ambient teardown on ended_network matching current_network, not just the final current_network clear.