Split pane_group orchestration code into submodules

[cephalonaut]

Description

Follow-up to PR #11722 (orch-restore Phase 1) addressing Roland's review nit that pane_group/mod.rs is too long (~9k lines).

Mechanical move only — no behavioral changes. Moves orchestration-specific code into three new submodules under app/src/pane_group/:

• ambient_pane_restoration.rs (new) — top-level ambient pane pending-restoration: AmbientRestoreKind, register_pending_ambient_restorations, process_pending_ambient_restorations, replace_pane_with_new_cloud_conversation, fetch_and_load_transcript.
• child_agent/hydration.rs (new) — remote-child cloud-transcript hydration: RemoteChildHydrationAction, decide_remote_child_hydration_action, hydrate_task_backed_hidden_child_pane, attempt_remote_child_hydration, apply_existing_ambient_task_to_pane, hydrate_remote_child_transcript_in_place, attach_ambient_session_and_maybe_tombstone, process_pending_remote_child_hydrations.
• child_agent/restoration.rs (new) — hidden child pane materialization: create_hidden_child_agent_pane, restore_missing_child_agent_panes_for_parent, restore_missing_child_agent_panes_for_terminal_pane_if_needed, ensure_hidden_child_agent_pane_for_conversation, ensure_shared_session_viewer_child_pane.

mod.rs retains the shared subscription wiring (ensure_pending_ambient_restoration_subscription, handle_pending_ambient_restoration_event) and all PaneGroup struct fields. Line count drops from 8946 → 7996.

pane_group/child_agent.rs was converted to a directory (child_agent/mod.rs) in commit 1 (pure rename); commit 2 adds the new submodules and removes the moved code from mod.rs.

Testing

• cargo fmt --all -- --check
• cargo clippy -p warp --all-targets --features local_fs -- -D warnings
• cargo nextest run -p warp --features local_fs 'pane_group::' — 106/106 tests pass

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

Conversation: https://staging.warp.dev/conversation/d3b66223-bbbf-4a8f-9e12-7b31b9d5202e
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@cephalonaut

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Overview

This PR splits the pane_group orchestration restoration code into ambient pane restoration, remote child hydration, and child restoration submodules while keeping the existing PaneGroup orchestration behavior intact.

Concerns

• No blocking concerns found in the annotated diff.
• The spec context attachment reports that no approved or repository spec context was found, so there was no material implementation-vs-spec drift to evaluate.

Verdict

Found: 0 critical, 0 important, 0 suggestions

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[seemeroland]

Will review once your other PR is merged and this is rebased, since this is containing that PR's changes currently

[cephalonaut]

Sorry for the noise - should be rebased now

[seemeroland]

Nice, thank you!