What does this PR do?

• Fixes {env:...} variable substitution inconsistently fails for specific MCP server URLs #5299
• Fixes OpenCode crashes on variable substitution if the value contains replacement patterns like $` #11793

This PR fixes an issue where files containing $ would break opencode when your config used the {file:x} pattern. This is because String.prototype.replace treats $ as a replacement character, which is not what we want in this use case. This PR instead uses a replacement function, so that .replace does a "dumb" string replacement.

There is an alternative PR: #11795 . I respectfully believe that that PR's approach is flawed as it tries to escape the $, instead of just treating the $ as-is.

How did you verify your code works?

# verified that this test fails without my fix
cd packages/opencode && bun test test/config/config.test.ts

# verified that this fix works in my project where I hit this bug
bun dev -- ~/my-proj-where-i-found-this-bug