fix: enable token substitution in OPENCODE_CONFIG_CONTENT (take #)

[ariane-emory]

What does this PR do?

Route OPENCODE_CONFIG_CONTENT through load() to enable {env:} and {file:} token substitution. Uses the env var name as the path for clearer error messages instead of a generic placeholder.

Fixes anommalyco#13219

How did you verify your code works?

Manual testing, bun typecheck, bun test.

You may manually test by running commands akin to the following (assuming bash):

echo "matrix" > /tmp/test-secret.txt 
export MODEL="opencode/big-pickle"
OPENCODE_CONFIG_CONTENT='{"$schema":"https://opencode.ai/config.json","theme":"{file:/tmp/test-secret.txt}", "model":"{env:MODEL}"}' bun dev```

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate PRs Found

1. PR fix: token substitution in OPENCODE_CONFIG_CONTENT #13241 - fix: token substitution in OPENCODE_CONFIG_CONTENT

   • fix: token substitution in OPENCODE_CONFIG_CONTENT #13241

2. PR fix(config): apply token substitution for OPENCODE_CONFIG_CONTENT #13226 - fix(config): apply token substitution for OPENCODE_CONFIG_CONTENT

   • fix(config): apply token substitution for OPENCODE_CONFIG_CONTENT #13226

Why they might be related: Both PRs directly address the same issue (enabling token substitution in OPENCODE_CONFIG_CONTENT) that your PR #13349 is fixing. They appear to be previous attempts or related fixes for the same underlying problem mentioned in issue #13219.