Redesign party-mode skill as conversation-first, subagents opt-in (#2441)

Reframes the party-mode skill around outcomes instead of a rigid
subagent script, addressing issue #2280.

- Voicing the room is the default; subagents become opt-in (--subagents)
  for rounds that genuinely need independent thinking.
- Removes the lossy 400-word context cap and the 'Do NOT use tools'
  constraint that drove subagents to fabricate on grounded questions.
- Adds a 'What Good Feels Like' bar: short in-character turns, real
  drama over consensus, brevity by default.
- Adds orchestrator weaving so independently-produced turns read as one
  conversation without altering what any persona actually argued.
- Slims SKILL.md from 128 to 75 lines.