[junhsss]

Stacked on #137.

• Ctrl-C cancels the running command and exits 130. After it, the CLI sends nothing but the cancel, and a second Ctrl-C stops waiting. A command the CLI stops waiting for prints its execution id.
• cancel exits 0 once the execution has stopped.
• New sessions default to 15 minutes, the longest every plan allows. The API's 5-minute default ended sessions before the 10-minute inactivity timeout could fire.
• eval --timeout, range checks on wait, masked values print ••••••••, notes when a scroll or back does nothing, navigation failures no longer mention input, and pdf help no longer says headless only.