feat(executor): region-qualified model prefixes and -auto/ qualifier

[favoroo]

Summary

Adds explicit region control over model-name prefixes for multi-region providers (WorkBuddy, Qoder).

• workbuddy-cn/model / workbuddy-gl/model route to CN or Global accounts.
• A bare provider prefix (qoder/…, workbuddy/…) is unchanged: no region is pinned and the pool auto-picks across regions, so international-only deployments keep working.
• New -auto/ qualifier (qoder-auto/…, workbuddy-auto/…): kept as an explicit spelling of the bare-prefix behavior (provider pinned, region auto-picked, session-affinity latch reused). Chosen over deleting it so users can write the intent explicitly; it is documented as equivalent to the bare prefix.
• Threads the model-name region through the route loop (WithRegionFilter), overriding key-grant and pin-derived regions while still enforcing both filters independently.

Review follow-ups (2026-10-19)

• Dropped the bare-prefix-defaults-to-CN commit (defaultRegionForProvider is gone); bare prefix and -auto/ are now equivalent.
• compat_test.go, s01_protocol_contract_test.go, regression_test.go expectations are back to their original form.
• New tests: TestPickRouteBarePrefixMatchesGlobalOnlyPool (a global-only pool still routes a bare qoder/model; an explicit -cn/ pin does not match it) and TestPrepareBarePrefixSetsNoRegionFilter.
• Changelog fragment reworded accordingly; rebased onto latest main.

Tests

• go build ./..., go vet ./..., go test ./..., git diff --check
• Executor prepare/route and API contract tests updated.

[caigee-cmd]

感谢提交！显式区域前缀（workbuddy-cn/、workbuddy-gl/）以及 -auto/ 这个思路很好，把区域贯穿到路由循环（WithRegionFilter）、同时仍分别校验 key 授权和区域这一点也处理得很清楚。

合并前需要调整的核心问题：

1. 不带区域的前缀（qoder/、workbuddy/）不能默认锁定为 cn。很多用户只有国际账号，合并后他们现有的 qoder/xxx 和 workbuddy/xxx 请求会直接找不到可用账号，属于破坏性变更。请保持不带区域的前缀行为不变，继续自动选区；-cn/ 和 -gl/ 只作为可选的显式限定。具体来说就是去掉 defaultRegionForProvider，或者让它默认为空。
2. 这样一来 -auto/ 和不带区域的前缀行为一致。可以保留它作为显式写法，也可以删掉以减少概念，两种都可以接受，请在 PR 描述里说明你的选择。
3. 请把 compat_test.go、s01_protocol_contract_test.go、regression_test.go 中因默认 cn 而改动的预期恢复原样，并补一个用例：只有国际账号时，qoder/model 仍能路由成功。
4. 请同步更新 changelog 片段中关于默认 cn 的描述，并 rebase 到最新 main。

改完后 @ 我一下即可，谢谢！

[favoroo]

@caigee-cmd 已按 review 调整完毕：

1. 裸前缀不再默认 cn：defaultRegionForProvider 已移除（整个提交从分支里 drop 掉），qoder/…、workbuddy/… 保持原有行为——不锁定区域，由池自动选区，只有国际账号的部署不受影响。
2. -auto/ 保留：作为裸前缀行为的显式写法保留（两者语义一致），已在 PR 描述和 changelog 里说明这一选择。如果你更倾向删掉以减少概念，我再移除。
3. 测试：compat_test.go、s01_protocol_contract_test.go、regression_test.go 的预期随默认 cn 的移除自动恢复原样（这三份文件现在与 main 无 diff）。新增两个用例：TestPickRouteBarePrefixMatchesGlobalOnlyPool（只有国际账号时裸 qoder/model 仍能路由成功，且显式 -cn/ 不会误匹配）和 TestPrepareBarePrefixSetsNoRegionFilter。
4. changelog 中关于默认 cn 的描述已改写；分支已 rebase 到最新 main，go test ./... 全绿。

麻烦再看一下，谢谢！

[caigee-cmd]

LGTM. Bare prefixes keep auto region selection; -cn/-gl/-auto work as documented; tests and changelog updated. Merging.