feat: merge default config files

[haosenwang1018]

Summary

• merge all discovered default mcp_servers.json files instead of stopping at the first hit
• keep explicit -c/--config and MCP_CONFIG_PATH behavior unchanged (single file)
• add tests for merged default configs and same-name override precedence

Problem

Issue #28 points out that mcp-cli currently stops at the first config file it finds. That makes it impossible to combine a project-local MCP setup with personal/user-level MCP servers.

Fix

When no explicit config path is provided, loadConfig() now loads every existing default config path and merges them from lowest priority to highest priority:

• ~/.config/mcp/mcp_servers.json
• ~/.mcp_servers.json
• ./mcp_servers.json

This preserves the current precedence model while allowing project configs to extend or override user configs. Explicit -c/--config and MCP_CONFIG_PATH still load exactly one file, as before.

Fixes #28

Validation

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