fix(installer): use opencode config dir on Windows

[NgoQuocViet2001]

Summary

• Route global opencode installs to ~/.config/opencode on Windows instead of %APPDATA%/opencode.
• Keep the existing XDG_CONFIG_HOME override behavior for global installs.
• Add a Windows regression test that sets APPDATA and verifies CodeGraph writes both opencode.jsonc and AGENTS.md under ~/.config/opencode.
• Make one existing local opencode path assertion separator-agnostic so the installer target test runs cleanly on Windows.

Closes #186.

Why

OpenCode documents the global config path as ~/.config/opencode/opencode.json, and #186 reports that the installer-created %APPDATA%/opencode config is not the file OpenCode reads on Windows. That makes codegraph install --target=opencode appear successful while the MCP server and instructions do not load.

Duplicate check

Checked recent/open PRs for opencode. #162 and #163 already merged the multi-target installer and .jsonc/AGENTS.md support, but they do not address the Windows %APPDATA% path. I did not find an open PR covering #186.

Test plan

• node node_modules/vitest/vitest.mjs run __tests__/installer-targets.test.ts — 57 passed
• node node_modules/typescript/bin/tsc --noEmit

[skywalker-35]

Confirmed this fixes a real bug — hit it today on Windows with OpenCode v1.15.10 and CodeGraph v0.9.4. The installer's %APPDATA%\opencode\ output looks successful but OpenCode silently ignores it. Had to manually move config to ~\.config\opencode\ to get the MCP server loading.

Tests pass cleanly, and this is blocking every Windows + OpenCode user from a working install. Any chance this can be merged soon?

[NgoQuocViet2001]

Thanks for confirming, @skywalker-35. I rebased this on the latest main and resolved the test conflict; GitHub now shows the PR as mergeable again.

Validation:

• node node_modules/vitest/vitest.mjs run __tests__/installer-targets.test.ts (83 passed)
• node node_modules/typescript/bin/tsc --noEmit

@colbymchenry this should be ready for another look/merge when you have a chance.

[accodev]

Will this be merged soon? Thank you :)