[codex] Add Toss OpenAPI broker support

[smallfish06]

Summary

• Add Toss as a built-in broker with common broker API mappings and a generated-spec raw proxy.
• Add Toss OAuth/client/rate-limit/error handling, public pkg/toss, and specgen targets.
• Extend config and common broker types for Toss env credentials, account sequence, fractional positions, and Toss order options.
• Add Toss live-smoke coverage and update docs/examples.

Validation

• go test ./...
• make toss-spec-check
• make build
• git diff --check
• Toss live smoke: PASS=10 FAIL=0 (auth, accounts, prices, stocks, holdings, GetQuote, GetOHLCV, GetInstrument, GetBalance, GetPositions)