feat: Add conversations list command

[edenreich]

Adds a new infer conversations list command to view saved conversation history with pagination support. Includes text and JSON output formats, cost formatting, and improved gateway startup handling. The command shows conversation metadata including token usage and costs.

Key Changes:

1. New command: infer conversations list with --limit, --offset, and --format flags
2. Documentation: Updated CLAUDE.md and README.md with command usage
3. Gateway improvements: Added EnsureStarted() method for consistent gateway startup
4. Cost formatting: New FormatCost() helper with adaptive precision
5. Tests: Comprehensive unit tests for the new functionality

Usage Examples:

# List conversations with default settings
infer conversations list

# List with pagination
infer conversations list --limit 10 --offset 20

# Output in JSON format
infer conversations list --format json

# Show only 5 most recent conversations
infer conversations list --limit 5

Technical Details:

• Uses the existing conversation storage system
• Supports both text (table) and JSON output formats
• Includes proper error handling and validation
• Follows existing code patterns and conventions

This feature enables users to easily browse and manage their conversation history, which is particularly useful for tracking usage and costs over time.

Closes #330

[ig-semantic-release-bot]

🎉 This PR is included in version 0.87.0 🎉

The release is available on GitHub release

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