feat(opencode): Added {env:VAR} substitution to markdown-based config parsing

[Eric-Guo]

Close #5054

The key difference between this PR and #5092 is this PR allow ENV usage in markdown body.

I already including a real use case in document and give the warning of security concerns.

Notice current json configuration of prompt allow using ENV, so I think using ENV in markdown body should also allowed.

Other coding agent like kimi-cli using ENV in their system prompt, so security concerns is not as serious as the first sight.