Validate Fledgling config and avoid stale module-level config state

[nick-pape]

Summary

Fledgling config is parsed with a direct cast and cached in a module-level promise, so invalid config shape is not diagnosed clearly and config cannot be reloaded during a process lifetime.

Evidence

• packages/acp-agent/src/config.ts: loadConfig() uses JSON.parse(...) as FledglingConfig without schema validation.
• packages/acp-agent/src/mcp-session-tools.ts: configPromise is initialized at module load.

Expected Work

• Add schema validation for config files.
• Produce useful diagnostics for invalid MCP server entries.
• Decide whether config should be loaded once or per session.
• Add tests for valid config, invalid config, missing config, and duplicate server names.