feat(providers): add Step Plan support

Document how to use StepFun's Step Plan subscription endpoint with the
existing `stepfun` provider by overriding `apiBase`, following the same
pattern as the `zhipu` provider's coding plan documentation.

- **Base URL**: `https://api.stepfun.com/step_plan/v1` (dedicated endpoint)
- **API Key**: same `STEPFUN_API_KEY` as the regular `stepfun` provider
- **Models**: `step-3.5-flash`, `step-3.5-flash-2603`, `step-router-v1`

Changes:
- `docs/configuration.md` — provider tip, and config example showing
  `apiBase` override on the existing `stepfun` provider

Test: 488/488 provider tests passed.

docs/configuration.md

@@ -126,6 +126,7 @@ ANTHROPIC_API_KEY="$(bw get password api/anthropic)" nanobot agent
 > - **VolcEngine / BytePlus Coding Plan**: Use dedicated providers `volcengineCodingPlan` or `byteplusCodingPlan` instead of the pay-per-use `volcengine` / `byteplus` providers.
 > - **Zhipu Coding Plan**: If you're on Zhipu's coding plan, set `"apiBase": "https://open.bigmodel.cn/api/coding/paas/v4"` in your zhipu provider config.
 > - **Alibaba Cloud BaiLian**: If you're using Alibaba Cloud BaiLian's OpenAI-compatible endpoint, set `"apiBase": "https://dashscope.aliyuncs.com/compatible-mode/v1"` in your dashscope provider config.
+> - **StepFun Step Plan**: If you're on StepFun's Step Plan subscription, set `"apiBase": "https://api.stepfun.com/step_plan/v1"` in your stepfun provider config. Supported models include `step-3.5-flash`, `step-3.5-flash-2603`, and `step-router-v1`.
 > - **Step Fun (Mainland China)**: If your API key is from Step Fun's mainland China platform (stepfun.com), set `"apiBase": "https://api.stepfun.com/v1"` in your stepfun provider config.
 > - **Xiaomi MiMo thinking mode**: MiMo models (e.g. `mimo-v2.5-pro`) default to enabled thinking. Use `agents.defaults.reasoningEffort: "none"` to disable it, or `"low"` / `"medium"` / `"high"` to keep it on. Omitting the field preserves the provider's per-model default.
 > - **Xiaomi MiMo Token Plan**: If you're on MiMo's token plan, set `"apiBase": "https://token-plan-sgp.xiaomimimo.com/v1"` in your xiaomi_mimo provider config.
@@ -548,6 +549,35 @@ the model name contains `mimo`. The default API base is
 
 </details>
 
+<details>
+<summary><b>StepFun Step Plan (subscription)</b></summary>
+
+Step Plan is StepFun's subscription-based service for high-frequency AI developers.
+If you're on a Step Plan subscription, override `apiBase` in the existing `stepfun`
+provider config to point to the dedicated Step Plan endpoint.
+
+```json
+{
+  "providers": {
+    "stepfun": {
+      "apiKey": "${STEPFUN_API_KEY}",
+      "apiBase": "https://api.stepfun.com/step_plan/v1"
+    }
+  },
+  "agents": {
+    "defaults": {
+      "provider": "stepfun",
+      "model": "step-3.5-flash"
+    }
+  }
+}
+```
+
+Supported models include `step-3.5-flash`, `step-3.5-flash-2603`, and
+`step-router-v1`.
+
+</details>
+
 <details>
 <summary><b>Ant Ling (OpenAI-compatible)</b></summary>