Unify fork harness selection

[edwin-zvs]

Summary

• route both same- and cross-harness forks through one harness picker
• prefill the picker with the source harness so Enter forks without an initial prompt
• remove the duplicate shifted fork shortcuts and update help/specs

Tests

• cargo test -p agentd-cli fork_picker_defaults_to_source_harness_without_prompt_stage -- --nocapture
• cargo test -p agentd-cli shifted_fork_shortcut_is_removed -- --nocapture
• cargo test -p agentd-cli (838 passed; timing-sensitive many_small_events_stay_fast_as_history_grows failed in the full run, then passed in isolation)
• cargo test -p agentd-cli pty_render::tests::many_small_events_stay_fast_as_history_grows -- --nocapture
• cargo build

Visuals

Not recorded: this reuses the existing harness picker UI and changes its default value/keybinding behavior.