fix: resolve opencode via shutil.which and pipe prompts through stdin on Windows

[dopamine-pixels]

Problem

desloppify review --run-batches --runner opencode fails on Windows when opencode is npm-installed. The runner invokes the bare name opencode, which CreateProcess cannot resolve for the npm .cmd shim — every batch exits 127 with [WinError 2] The system cannot find the file specified, and the run reports "Runner CLI not found on PATH" even though opencode is on PATH.

Fix

Mirror the existing codex runner handling:

• Resolve the executable with shutil.which and, on Windows, wrap .cmd/.bat shims in cmd /c.
• Because cmd.exe re-parses argv and mangles prompts containing quotes or angle brackets, the prompt is sent via stdin whenever the cmd /c wrapper is used (the
  un_batch_attempt plumbing already supported stdin). .exe binaries and non-Windows platforms keep argv prompt passing.

Verification

• Verified against a real project on Windows 11: batch prompts now execute and return JSON payloads (opencode run --format json NDJSON stream extracted and persisted).
• Added 4 unit tests (shim wrap + stdin, direct exe, non-Windows argv, stdin plumbing through
  un_opencode_batch).
• pytest desloppify/tests/commands/review/test_review_runner_helpers_direct.py — 14 passed.
• Full suite: failure set identical to base commit (31 pre-existing Windows-environment failures, no regressions).