feat(cli): add built-in help command for REPL

[aviralgarg05]

Summary

Add CLI help command(s) for first-time users.

Why

Discoverability is low without built-in guidance for available commands and SQL examples.

Tasks

• Add help (or .help) command in nexum_cli/src/main.rs
• Print available commands: SQL mode, ASK, EXPLAIN, exit options
• Include 2-3 short query examples

Acceptance Criteria

• Help command works in REPL
• Output is concise and readable
• Existing CLI behavior remains unchanged

[coderabbitai]

🔗 Related PRs

#101 - feat(cli): add colorized output and improved formatting [merged]

---

📝 Issue Planner

_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

• Create Plan

---

🧪 Issue enrichment is currently in open beta.

You can configure auto-planning by selecting labels in the issue_enrichment configuration.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!

[SiriVennela24-star]

Heyy @aviralgarg05 ,i would like to add those cli commands. Could you please assign me this issue ? I would like to work on this issue.