v0.2.1 — OpenCode Zen provider

New provider: OpenCode Zen (https://opencode.ai/zen/v1/systemone, default checkpoint jev-1.13, credential OPENCODE_ZEN_API_KEY). Wire-compatible with the TypeSafe contract; the free checkpoint is not usable from DSH (403 from outside the OpenCode client) and an unfunded account answers 402 — both documented in code.

• README provider table updated with the Zen row
• 51/51 tests passing