Fold clap into figment as the top layer

[ecalifornica]

Part of #183, phase 3. Blocked on #188.

• Config-type flags join the figment merge as the top layer (--pretty, --picker, share --url).
• Positional arguments and subcommand data (session ids, file paths, resume <input>) are data, not config. They stay out of figment.
• Below run(), no code knows whether a value came from flag, env, or toml.
• Revisit here: should any diagnostic env vars become flags?

[ecalifornica]

Include fuzzy::set_picker_override here: it is the one existing write-once global (set in run(), read later by the fuzzy module). --picker is a config-type flag, so this phase converts it to parameter passing, per the #183 decision: no global store.