docs(sdk): add MCP server env variable resolution for file-based agents

[VascoSch92]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

• Document ${VAR} environment variable resolution in MCP server configs for file-based agents, following feat(sdk/subagent): Resolve ${VAR} env variables in all MCP server config fields  software-agent-sdk#2492

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Project	Status	Preview	Updated (UTC)
all-hands-ai	🟢 Ready	View Preview	Mar 25, 2026, 5:55 PM

[all-hands-bot]

🟢 Good taste - Clean, accurate documentation that solves a real problem.

Verified against upstream implementation in software-agent-sdk PR #2492. All claims match the actual code:

• Environment variable resolution uses os.path.expandvars() with both ${VAR} and $VAR syntax
• Recursively processes all string values in MCP server configs
• Undefined variables are left unchanged as documented
• Example shows realistic usage across different server types

This documents how users can avoid hard-coding secrets in agent definitions - straightforward and pragmatic. No issues found.

[xingyaoww]

Thank you!