Add per-show playback control to shows create

[aboullaite]

What

Shows can now opt into chapter-skip playback at creation time: the player's skip forward/back buttons jump between chapters instead of seeking 15 seconds.

save-to-spotify --json shows create --title "My Show" --playback-control chapter-skip

How

• shows create gains --playback-control <mode>. Modes live in a small registry (CLI name → API value), so future modes are one map entry. Unknown modes are rejected at parse time, before any network call — the backend silently drops values it doesn't recognize, so a typo passed through would create a default-controls show with no error.
• The mode is sent as an optional playback_control field on the existing show-create request; omitted entirely when unset, so normal creates are unchanged.
• The create response echoes playback_control only when the backend applied the setting. The CLI surfaces that server-confirmed value (mapped back to the CLI mode name) and prints a warning when the setting was requested but not confirmed; in --json mode the field's absence is the machine-readable signal.
• New configure-chapter-skip skill owns when to enable the setting (explicit user request only) and the workflow; the main skill and CLI reference point to it.

Testing

• Unit + integration tests cover flag parsing, wire format (present/omitted), the confirmed echo, and the not-confirmed case.
• Verified end to end against the live API: created shows with and without the flag, confirmed the response echo, and validated playback on real episodes with chapters.