fix(server): resolve Kimi provider lint and typecheck failures

[tarikermis]

Refs pingdotgg#5243

What changed

This fixes the lint and typecheck failures currently blocking the Kimi provider branch in apps/server:

• Replace direct process.platform checks in the Kimi tests with the repository's isHostWindows abstraction.
• Remove the unused EMPTY_CAPABILITIES constant.
• Replace the removed Schema.UnknownFromJsonString API with Schema.fromJsonString(Schema.Unknown), matching the Claude and Grok adapters.
• Drop obsolete ["Service"] indexing from Effect filesystem and path types, and let writeFakeKimiBinary infer its real error channel.
• Suppress preferSchemaOverJson only for the JSON.stringify call that writes the mock-agent JavaScript shim.

These are compatibility and test-helper fixes only; provider behavior is unchanged.

Verification

Based on the current pingdotgg#5243 head (a0b60ca4e):

• vp check passes.
• tsgo --noEmit passes in apps/server; only existing suggestions remain in src/orchestration/decider.ts.
• All 19 Kimi provider tests pass across the four affected test files.

Windows paths are typechecked but were not run locally. The upstream workflow is still waiting for external-contributor approval, so the evidence here is from the equivalent local gates rather than a GitHub CI run.

I used Kimi Code for the implementation and Claude for a second review.

Checklist

• This PR is small and focused.
• I explained what changed and why.
• I included before/after screenshots for UI changes. (N/A)
• I included a video for interaction changes. (N/A)

Best Regards, Tarik

[tarikermis]

Closing this since the upstream Kimi provider branch is no longer active after pingdotgg#5243 was closed. This patch only fixed lint and type errors on that feature branch, so there is nothing useful to rebase it onto now.