Studio Agents Suite: Add multisite info to agents.md

[katinthehatsite]

Related issues

N/A

How AI was used in this PR

Not used, I updated the file manually

Proposed Changes

Now that #2470 was merged, we support multisite when the site was added from a blueprint that contains multisite. This PR updates the AGENTS.md file shipped at the root of Studio sites to include this information.

Testing Instructions

• Visual review should suffice

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wpmobilebot]

📊 Performance Test Results

Comparing cfb16ff vs trunk

site-editor

Metric	trunk	cfb16ff	Diff	Change
load	1774.00 ms	1774.00 ms	0.00 ms	⚪ 0.0%

site-startup

Metric	trunk	cfb16ff	Diff	Change
siteCreation	6102.00 ms	6112.00 ms	+10.00 ms	⚪ 0.0%
siteStartup	3937.00 ms	3927.00 ms	-10.00 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[epeicher]

Thanks @katinthehatsite, the wording explaining the Multisite functionality makes sense to me, and it is also discovered by Claude. LGTM! 👍

[bcotrim]

LGTM, thanks for catching this and making the change 👍