Implement devlog doctor for local setup and configuration diagnostics

[gustmrg]

Add a devlog doctor command to validate local setup, configuration, and common runtime prerequisites.

Context:

• DevLog depends on a local filesystem layout under ~/.devlog/
• Summary generation depends on valid config and, when enabled, LLM-related environment variables
• A diagnostic command would make onboarding, support, and troubleshooting much easier

Expected behavior:

• Run a set of health checks and print a human-readable report
• Clearly distinguish:
  • successful checks
  • warnings
  • failures
• Exit non-zero when critical problems are found
  Checks should include:
• ~/.devlog/ exists
• config.json exists and can be parsed
• entries/, summaries/, and templates/ directories exist
• core config keys are present and valid
• if llm.enabled=true, the configured API key environment variable is set
• optionally validate that stored summary files and entry files are readable
  Possible enhancements:
• --fix to create missing directories or repair safe, trivial issues
• --json for machine-readable output
• --verbose for more detailed diagnostics

Implementation notes:

• Reuse existing config and store utilities where possible
• Keep the first version focused on safe read-only checks unless --fix is explicitly used
• Output should be actionable, not just descriptive

Acceptance criteria:

• devlog doctor exists and appears in help output
• It detects missing config or directories
• It reports invalid config or missing required environment variables
• It exits with a failing status when critical setup issues are found
• Output gives the user clear next steps to resolve problems