fix(core): apply variable substitution to v2 config loading

[vladislav-miroshnikov]

Issue for this PR

Closes #41973

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The v2 config loading path in packages/core/src/config.ts reads config files and parses them as JSONC without applying {file:} and {env:} variable substitution. The v1 path in packages/opencode/src/config/config.ts calls ConfigVariable.substitute() on the raw text before parsing. Through opencode run and the TUI (v1 path), {file:/tmp/demo.key} is resolved and the provider authenticates. Through opencode serve (v2 path), the literal string {file:/tmp/demo.key} ends up as the API key, causing 401s.

This PR adds an Effect-based substitute() function in packages/core/src/config/variable.ts that handles {env:VAR} and {file:path} substitutions, and calls it in loadFile() before JSONC parsing. The implementation mirrors the v1 substitution logic: supports ~/ expansion, relative paths (relative to the config file), skips substitution inside // line comments, and trims file content.

How did you verify your code works?

Added 5 tests to config.test.ts covering: {file:} with relative paths, {env:} substitution, // comment skipping, {file:} with absolute paths, and {file:} content containing JSON-special characters (quotes, backslashes, newlines). 20/20 tests pass, typecheck clean, prettier clean.

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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