Enhance `logs` command with summary, failures, and JSON output

[Miyamura80]

Problem

desktest logs supports --steps ranges for viewing trajectory logs, but lacks filtering and output format options that would make it more useful for quick diagnosis and tooling integration.

Proposal

Add the following flags to desktest logs:

• --summary — One-line-per-step overview showing the action taken and success/fail status:

  Step 1: screenshot           ✓
  Step 2: click(450, 320)      ✓
  Step 3: type("hello")        ✗ (timeout)
  

• --failures — Show only failed steps, useful for quick diagnosis of long runs
• --json — Output structured JSON for piping to other tools (jq, dashboards, CI reporters)

Notes

• These are additive flags on the existing logs subcommand
• --summary and --failures can be combined
• --json output should include step number, action, result, duration, and any error message