Keep TUI hydration running after selection changes

[edwin-zvs]

Summary

• keep selected-session hydration tasks running when the user switches away
• cache stale hydration results for history/editor/status without replacing the visible transcript
• keep pinned hydration deduped against any selected hydration already in flight
• add regression coverage for stale selected hydration warming the history cache

Verification

• cargo test -p agentd-cli hydration
• cargo test -p agentd-cli terminal_view_rehydrates_after_history_dropped
• cargo build --workspace --all-targets --locked
• cargo test --workspace --locked -- --skip pty_render::tests::codex_initial_render_after_restart_is_bounded --skip pty_render::tests::many_pinned_sessions_resize_within_budget --skip pty_render::tests::pseudo_terminal_render_is_fast_per_frame --skip pty_render::tests::codex_resize_bound_is_reasonable
• git diff --check

Relevant code is in crates/cli, so the binary to try is target/debug/agent.