feat(ai): switch xiaomi default to api billing, add per-region token plan providers (#4112)

Built-in `xiaomi` provider now targets the API billing endpoint (https://api.xiaomimimo.com/anthropic) — a single stable URL for keys issued at platform.xiaomimimo.com. The Token Plan endpoints are exposed as three sibling providers, each with its own env var:

- xiaomi-token-plan-cn: XIAOMI_TOKEN_PLAN_CN_API_KEY
- xiaomi-token-plan-ams: XIAOMI_TOKEN_PLAN_AMS_API_KEY
- xiaomi-token-plan-sgp: XIAOMI_TOKEN_PLAN_SGP_API_KEY

BREAKING CHANGE: users who previously set XIAOMI_API_KEY against the Token Plan AMS endpoint must move to xiaomi-token-plan-ams and set XIAOMI_TOKEN_PLAN_AMS_API_KEY. This also resolves the 401 reported by on #4005, where a platform.xiaomimimo.com key fails against the Token Plan endpoint.

closes #4082

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,14 @@
 
 ## [Unreleased]
 
+### Breaking Changes
+
+- Switched the built-in `xiaomi` provider from Token Plan AMS to Xiaomi's API billing endpoint, and renamed its `/login` display from "Xiaomi MiMo Token Plan" to "Xiaomi MiMo". `XIAOMI_API_KEY` now refers to the API billing key from [platform.xiaomimimo.com](https://platform.xiaomimimo.com). Users on Token Plan should switch to the appropriate `xiaomi-token-plan-*` provider and set the corresponding env var.
+
+### Added
+
+- Added three Xiaomi MiMo Token Plan regional providers visible in `/login`: `xiaomi-token-plan-cn` (`XIAOMI_TOKEN_PLAN_CN_API_KEY`), `xiaomi-token-plan-ams` (`XIAOMI_TOKEN_PLAN_AMS_API_KEY`), `xiaomi-token-plan-sgp` (`XIAOMI_TOKEN_PLAN_SGP_API_KEY`). Each defaults to `mimo-v2.5-pro`.
+
 ### Changed
 
 - Changed `read` tool rendering to collapse Pi documentation, AGENTS/CLAUDE context files, and `SKILL.md` contents by default in interactive output.