fix(cli): expose MCP configuration diagnostics in doctor

[kingsword09]

zcode doctor previously showed runtime metadata without configuration diagnostics, and the launcher could reject broken settings before doctor ran. For example, a valid JSON file containing ui.theme: "system" can cause the runtime to ignore the user settings and omit its custom MCP servers.

This change makes that failure inspectable:

• Add configuration paths, loaded/missing/invalid status, validated MCP names and sources, and field diagnostics to text output and the JSON configuration report.
• Run doctor before first-run settings creation and provider migration, so it works without login and with malformed settings. Return exit code 1 for invalid configuration or skipped MCP entries.
• Omit MCP connection values and JSON parse input fragments from the report. Inspect native configuration without starting MCP servers or sending model requests.
• Document setting.json as this package's MCP configuration file, the separate provider schema, native MCP precedence, migration behavior, and upstream/Desktop path differences.

The report covers user/project configuration; plugin and built-in MCP servers are added at session startup. It does not test connectivity. MCP discovery and merge behavior remain unchanged.

Related to #188. This improves diagnosis of the reported symptom; the original reporter's environment-specific root cause is still unconfirmed.

Validation completed locally on macOS arm64:

• bun run typecheck
• bun run test:unit — 753 passed
• bun test test/runtime/config-diagnostics.test.ts test/runtime/config-migration.test.ts test/runtime/launcher.test.ts — 15 passed
• bun scripts/check-runtime.ts
• Verified patch application and idempotency against the published 3.14.4-31 runtime.
• Connected to the real Context7 HTTP MCP using the native runtime client and successfully called resolve-library-id and query-docs.

[kingsword09]

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