fix: token substitution in OPENCODE_CONFIG_CONTENT

[ariane-emory]

What does this PR do?

• Route inline config through load() function to enable {env:} and {file:} token substitution
• Refactor load(...) to accept an object as its second parameter and update its call sites accordingly.
• Add tests for {env:} and {file:} token substitution in OPENCODE_CONFIG_CONTENT

NOTE: This PR is an alternate take on this issue. Unlike PR anomalyco#13384 (which was merged and subsequently reverted as it introduced a regression but about which I am thoroughly embarrassed). This alternate version changes one of the parameters of the load function in packages/opencode/src/config/config.ts to an object. While a slightly more complex change, this provides a more robust solution to the issue and avoids the possibility of passing {} as the environment variable's value creating an unwanted extra file in the project's root,

Fixes anomalyco#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
OPENCODE_CONFIG_CONTENT='{"theme":"{file:/tmp/test-secret.txt}", "model":"{env:MODEL}"}' bun dev # observe that it still works predictably without a $schema property on the object
OPENCODE_CONFIG_CONTENT='{}' bun dev # observe that no unwanted files are created