feat: name the background flag --daemon and add the leader help overlay

[whackur]

What

• -d is now --daemon instead of --detach. Detaching is what <prefix> q does to the TUI, so -d attach read as two opposite requests. The short flag is unchanged; child_args now strips the new spelling when re-execing the background copy.
• --exec, --port, --bind and --daemon are rejected on init, plugin, stop, status and update, which start no session and silently dropped them. attach still accepts them and hands them to the daemon it starts.
• <prefix> ?, or <prefix> h, opens a help overlay listing every command with its keys, matching the web viewer's shortcut sheet. It is workspace-level, so it opens with no project on screen. j/k, the arrows, PageUp/PageDown and Space scroll; Esc, Enter, q, ?, h and Ctrl+C close. Every other key is consumed while it is up.

Verification

cargo fmt --all --check, cargo build, cargo clippy --all-targets --all-features -- -D warnings pass. cargo test: 1948 pass, with platform::links::tests::immediate_editor_failure_is_reported_without_running_a_shell failing under full-suite load and passing on its own. That test re-execs the test binary and expects it to exit inside EDITOR_STARTUP_GRACE; it fails the same way on dev without these changes, so it is a pre-existing timing flake rather than a regression here.

The viewer bundle is untouched.