fix: validate connector config files

[HwangJohn]

Summary

• add connector id and config path context when connector config files cannot be read or parsed
• reject connector config JSON values that are not objects before merging with defaults
• keep the existing missing-file default config behavior

Why

A malformed or hand-edited ~/.openwiki/connectors/<id>/config.json could previously surface as a raw JSON syntax error, or arrays/primitives could be spread into runtime config values. This makes connector setup failures easier to diagnose and avoids silently accepting invalid config shapes.

No dedicated issue; found during a connector hardening sweep.

Testing

• corepack pnpm exec vitest run test/connector-config.test.ts passed
• corepack pnpm run format:check passed
• corepack pnpm run lint:check passed
• corepack pnpm run typecheck passed
• git diff --check passed
• corepack pnpm test was attempted on Windows and still fails outside this change in existing test/env-behavior.test.ts HOME/permission assertions plus Mermaid/jsdom timeout tests