fix: use current OpenCode paths on Windows

[iHildy]

Summary

• use USERPROFILE as the Windows home and match current OpenCode XDG defaults:
  • config: ~/.config/opencode
  • data: ~/.local/share/opencode
  • state: ~/.local/state/opencode
• preserve explicit XDG_CONFIG_HOME, XDG_DATA_HOME, and XDG_STATE_HOME overrides
• use explicit path.win32 semantics whenever Windows is injected, including sync-plan and repository paths
• add exact cross-host Windows regressions plus a real windows-latest path-test job without changing the existing Ubuntu check

This is a narrow replacement for the already-closed #68. It does not include that PR's package, repository, workflow-fork, portable-state, or frontier-selector changes.

Upstream evidence

• current upstream OpenCode uses xdg-basedir for config, data, and state roots
• local OpenCode 1.18.23 opencode debug paths reports .config/opencode, .local/share/opencode, and .local/state/opencode
• the plugin does not model OpenCode cache, bin, or log roots, so this PR does not add them

Verification

• focused path suite: 25 tests passed
• bun run check
• bun test: 107 tests passed, 233 assertions
• bun run build
• live windows-latest: 25 path tests and 60 assertions passed using Bun 1.3.2 against an authenticated source archive pinned to head 9acb43a8640382c1ff16c7d4f9acad75617b9349
• existing Ubuntu Check and PR-title validation passed
• full isolated GitHub E2E passed: machine A init and two pushes, machine B link and pull, source tree unchanged, and ephemeral repository deleted
• synthetic merge with updated fix: resolve relative extra paths from config root #72: 115 tests passed and build passed
• synthetic merge with repaired fix: portable paths in extra-manifest for cross-platform sync #58: 140 tests passed and build passed

The Windows job uses the exact-head GitHub source archive because Windows Git cannot materialize the repository's existing Unix-only .mise/tasks/lint:fix filename. The isolated harness emitted its known warnings: password-free loopback servers, duplicate sandbox skill discovery, and machine B's live AGENTS.md not refreshing even though its sync clone contained the second sentinel and confirmed repository replication.

Closes #59