feat: record start [path], screenshot [path], --record-json

[thymikee]

This PR improves output path ergonomics for media capture and session logs.

What changed

• Added positional output path support for screenshots: agent-device screenshot [path]
• Standardized recording start syntax with optional path: agent-device record start [path]
• Extended --record-json flow to support custom JSON output paths (including directory targets with auto-generated filenames).

Closes #15

[github-actions]

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