fix(omo): detect unified config on the WSL side when the OpenCode dir is in WSL

[fszcd]

Reopened from #7367 after branch restoration. @farion1231 already reviewed and approved the original at 2840692.

Root cause: when the OpenCode config directory lives inside WSL, the unified-config detection path only checks Windows-side paths and misses the WSL location. This fix adds WSL-side detection to the read/write/delete candidate paths.

Original PR: #7367

[fszcd]

@farion1231 Branch restored and resubmitted as this PR. The original #7367 could not be reopened because the head branch had been force-pushed after deletion. Content is identical to the approved commit 2840692. Ready for your review/merge.

[farion1231]

Merged as 37d0476 — thank you @fszcd, both for the fix and for restoring the branch and resubmitting so quickly.

Before merging I confirmed the patch is identical to what I reviewed on #7367 at 2840692 (same patch-id), and CI is green on all four backend jobs here as well. A few things I appreciated:

• Deriving the WSL home from the leading path segments (home/<user> / root) instead of walking up from the OpenCode dir. It stays correct for non-standard layouts, and it can't mis-derive a "home" for a non-WSL custom directory.
• Routing read, write and delete through the same candidate list, so the three paths can't drift apart again.
• Keeping the OS home as a fallback, which means the symlink workaround described in [Bug] WSL 下 OMO 统一配置 (~/.omo/omo.jsonc) 无法被 CC Switch 识别/写入：统一配置探测使用 Windows home #7363 keeps working for anyone who already set it up.
• The new tests really do execute on windows-latest — I checked the CI log rather than assuming, since most of them are cfg(windows).

One optional follow-up, only if you feel like it: derive_wsl_home_dir and derive_wsl_default_mcp_path now share about 25 lines of identical UNC-prefix parsing, which could move into a small shared helper. Not needed for this fix, and a separate small PR would be welcome.

#7363 was closed by this merge, and the fix will go out with the next release.