fix: scope env warnings to selected server

[haosenwang1018]

Summary

• scope config env substitution to the selected server when mcp-cli info <server> is used
• avoid unrelated missing-env warnings from other configured servers
• add regression coverage for scoped config loading

Problem

Issue #20 reports that mcp-cli info <server> emits missing environment variable warnings for every server in the config, even when only one server is being queried.

Fix

loadConfig() now supports scoping env substitution to a requested server subset. The info command passes the selected server name into config loading, so env placeholders are only expanded (and warned on) for the server actually being inspected. Commands that need the full config still retain the existing whole-config behavior.

Fixes #20

Validation

• bun test tests/config.test.ts
• bun test tests/output.test.ts
• npx tsc --noEmit
• npx @biomejs/biome check src/config.ts src/commands/info.ts tests/config.test.ts
• git diff --check