Add AntSeed wrapped agent commands

[gabrielpoca]

What

• Adds antseed codex, antseed claude, and antseed opencode wrapper commands.
• Resolves the buyer proxy URL from the active buyer.state.json, then config, with http://localhost:8377 as the fallback.
• Lets users override the proxy with --antseed-base-url and select the AntSeed service/model with --model or ANTSEED_MODEL where required.
• Forwards all other arguments to the wrapped tool, with -- available when child args should keep wrapper-looking flags.
• Updates the website integrations pages for Claude Code, Codex, and OpenCode to recommend the new wrapper commands while keeping manual setup as an alternative.

Tool behavior

• antseed codex injects per-run Codex -c config overrides for an AntSeed model provider using /v1 and wire_api = "responses", without writing to CODEX_HOME.
• antseed claude sets ANTHROPIC_BASE_URL to the proxy root and uses a placeholder ANTHROPIC_API_KEY.
• antseed opencode writes a temporary opencode.json custom provider config, points OPENCODE_CONFIG at it for the child process, and removes it after exit.

Examples

antseed codex --model minimax-m2.5
antseed codex --model minimax-m2.5 -- --version
antseed claude --model claude-sonnet-4-6
antseed opencode --model gpt-oss-120b run

Tests

• pnpm --filter=@antseed/cli run test
• pnpm --filter=@antseed/website run typecheck
• pnpm --filter=@antseed/website run build