feat: forge source test command

[alohays]

Summary

forge source test <name> — validates a source (especially cli-bridge) works before adding to config.

Usage

forge source test my-github-source
forge source test my-gmail-source --format json

Requirements

• Runs one fetch cycle for the named source
• Shows parsed output (or error details)
• Especially useful for cli-bridge sources to verify executable + args work
• Supports --format json for agent use

Acceptance Criteria

• forge source test <name> runs one fetch cycle
• Parsed output displayed on success
• Clear error details on failure
• Works with all source types (RSS, API, cli-bridge)
• Agent-compatible with --format json