[OPIK-5488] [FE] Update onboarding prompt to use /instrument and include workspace

[juanferrub]

Summary

• Updated the "Install with AI" onboarding prompt (step 2 of Get Started) to reference the /instrument command
• Added the active workspace name to the prompt so users' coding agents configure the correct workspace

Test plan

• Navigate to Get Started, enter an agent name, proceed to step 2
• Verify the prompt in the second code block includes /instrument, workspace name, project name, and API key
• Verify the copy button copies the full prompt with the real (unmasked) API key

🤖 Generated with Claude Code

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