v1.6.0 — CLI providers: debate on a subscription instead of API credits

CLI providers — debate on a subscription instead of API credits.

A third transport alongside API and hosted mode. brainstorm can now shell out to agent CLIs
installed on your machine, so debates run on a plan you already pay for.

• Any supported CLI found on your PATH is registered automatically at startup — no configuration
  needed. Use it as claude:sonnet, codex:default, and so on.
• Built-in adapters: claude and codex (verified), plus gemini, cursor-agent, opencode,
  qwen, kimi, and droid (modelled on vendor docs — flags may need adjusting; list_providers
  and the startup log say which is which).
• custom adapter runs any other CLI from an argv template with {{model}}, {{system}},
  {{prompt}}, and {{outfile}} placeholders.
• backend option points the Claude CLI at a vendor's Anthropic-compatible endpoint, so Moonshot
  (Kimi), MiniMax, and Z.ai (GLM) coding plans can join a debate. Tokens are read from the
  environment at call time and never stored in config.
• CLI calls run with tools disabled and a read-only sandbox where supported — they generate text,
  they don't touch your repo. Provider API-key env vars are stripped from the child process so the
  CLI falls back to subscription auth rather than billing credits.
• Cost estimates count CLI models as free: ~$0.0000 (2 of 3 via CLI subscriptions).

New env vars: BRAINSTORM_CLI_PROVIDERS (auto / off / adapter list), BRAINSTORM_PREFER_CLI,
BRAINSTORM_CLI_TIMEOUT_MS.

Also adds known base URLs and env-var detection for moonshot, minimax, glm, and qwen as
ordinary metered API providers.

add_provider accepts kind: "cli" with adapter / backend / command / args.
list_providers reports transport, verification status, and whether each CLI is on your PATH.