feat(cli): add --keep-workspaces flag and improve workspace summary

[christso]

Summary

• Add --keep-workspaces CLI flag to agentv eval run, wiring the existing orchestrator keepWorkspaces plumbing through the CLI
• Normalize with CLI > YAML config > TS config precedence (same pattern as --verbose)
• Improve end-of-run workspace summary: show preserved paths for all tests when --keep-workspaces is set, show hint when workspaces used without the flag
• Fix orchestrator bug: return workspacePath on result for successfully preserved workspaces (previously only failures included the path)

Red/Green UAT

Red (main):

• --keep-workspaces → Unknown arguments error
• Successful workspace paths never shown (orchestrator bug: workspace kept but path not in result)

Green (branch):

• --keep-workspaces accepted, workspaces preserved, paths shown for all tests
• Without --keep-workspaces: failed workspace paths shown + hint Use --keep-workspaces to preserve all workspaces for inspection.
• No hint when workspaces aren't used at all

Test plan

• Build passes
• All 2117 unit tests pass (0 failures)
• --keep-workspaces appears in --help output
• Red test: flag rejected on main
• Green test: flag accepted, workspace paths displayed, hint shown

Closes #1066

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