feat(cli): add doctor diagnostics

[wananing]

Summary

Add contextseek doctor as a CLI self-check for ContextSeek configuration and component connectivity.

The command loads ContextSeekSettings, reports component-level PASS / FAIL / SKIP results for config, storage,
embedding, and LLM, and returns a nonzero exit code when a configured required component fails.

Fixes #46.

Solution Description

Register a new doctor subcommand that runs before normal client initialization so configuration load errors can be
reported as diagnostics.

The diagnostics reuse the existing config factory path to resolve and probe components:

• storage is initialized and checked with a lightweight list operation
• embedding runs a tiny probe when an embedding provider is configured
• LLM runs a tiny prompt when an LLM provider is configured
• none embedding/LLM providers are reported as SKIP because they are optional in the default zero-config workflow

Diagnostic output includes actionable hints pointing users back to .env.example, redacts configured secrets from
summaries and hints, and exits with code 1 when any component reports FAIL.

Added unit tests for parser support, default diagnostics, misconfigured storage, resolved model reporting, secret
redaction, and nonzero failure behavior