fix: preflight the OpenCode PowerShell launcher

[CompleteDotTech]

Summary

Preflight the exact OpenCode launch plan and show unavailable runtimes truthfully in chat and setup UI.

Closes #3862.

Current implementation

• Uses the same familiar-scoped environment for Windows PowerShell host resolution, PATHEXT OpenCode lookup, capability probes, and the JSON-over-stdin spawn.
• Distinguishes missing OpenCode (runtime_missing) from a preflight-detected broken PowerShell host (runtime_unlaunchable); ambiguous Windows post-preflight ENOENT races remain value-free runtime_launch_failed.
• Prevents capability probes/spawns on unavailable plans; launch failures produce no auth/no-output diagnosis or fabricated assistant turn.
• Surfaces availability/remediation in Familiar Studio, Summoning Circle, and chat setup recovery without local path leakage.

Rebase and validation

Rebased onto current origin/main 14780987 and force-pushed head d4aba87d. The existing b0fe39cc follow-up fixes the branch-caused Frontend build failure: api-contracts.test.ts now pins the structured post-spawn race code instead of the obsolete ENOENT wire code, including the longer OpenCode classification branch.

Passed after this rebase: opencode-bin, runtime-availability, chat-send-capabilities, harness-routing-opencode, api-contracts, harnesses route, Chat View, Familiar Studio, Summoning Circle, and route-runtime-availability.integration; pnpm lint; pnpm codemod:design:check; pnpm exec tsc --noEmit --pretty false; and git diff --check.

Native Windows note: route-opencode.integration.test.ts cannot isolate its temporary shim on this workstation because the user-global opencode.cmd is rediscovered through the environment's Path entry; it therefore follows that global client rather than the fixture. The isolated CI environment covers the intended OpenCode and missing-runtime paths.