fix(opencode): support env-prefixed plugin paths in config

[himax12]

Fixes #25390

tui.json plugin specs using env/home-style prefixes were treated as package names instead of local paths.

What changed:

• expand ~, $VAR, ${VAR}, and %VAR% prefixes in plugin specs before path resolution
• keep package specs unchanged
• add regression tests for $HOME and %USERPROFILE%

Verification:

• bun --cwd packages/opencode test test/config/config.test.ts --timeout 60000
• passes locally (82 passed)

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[Copilot]

Pull request overview

This PR fixes config plugin resolution so that plugin specs in tui.json/config files starting with home/env prefixes (e.g. ~, $HOME, %USERPROFILE%) are expanded and treated as local paths instead of npm package names, and adds regression tests for the behavior.

Changes:

• Expand ~, $VAR, ${VAR}, and %VAR% prefixes in plugin specs before resolving them as paths.
• Preserve non-path (npm) plugin specs unchanged.
• Add tests covering $HOME/... and %USERPROFILE%/... directory plugin specs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/opencode/src/config/plugin.ts	Adds prefix-expansion logic and updates resolvePluginSpec() to treat expanded prefixes as path-like specs.
packages/opencode/test/config/config.test.ts	Adds regression tests ensuring $HOME and %USERPROFILE% prefixed plugin directory specs resolve to file URLs.

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[github-actions]

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