feat(tui): add first-run onboarding hints

[edwin-zvs]

Summary

• render a welcome empty state in the session detail pane when there are no visible sessions
• add a no-session modeline hint for create/help/palette shortcuts
• add a getting-started concepts section to the help modal

Tests

• cargo test -p agentd-cli empty_tui -- --nocapture
• cargo test -p agentd-cli help_modal -- --nocapture
• cargo test -p agentd-cli
• cargo build
• git diff --check

This change only touches crates/cli, so the relevant binary is agent.