feat(terminal): make automatic shell opt-in

[whackur]

Summary

• start projects without a terminal process by default
• add [terminal] auto_open = true to restore the previous automatic shell behavior
• preserve configured startup commands and --exec panes, including reload reporting
• document the new default and reload semantics

Verification

• cargo fmt --all --check
• cargo build
• cargo test (1801 passed, 4 ignored)
• cargo clippy --all-targets --all-features -- -D warnings