Agentic UI: Strip local domain suffix from publish flow domain suggestion

[bcotrim]

Related issues

• Fixes STU-2393

How AI was used in this PR

Written with Claude Code (analysis, implementation, tests); reviewed by me.

Proposed Changes

Following "Create a new WordPress site" from the publish/sync flow prefilled the WordPress.com domain search with the site's local domain (e.g. mysite.wp.local), so the first screen in the handoff showed an "invalid domain" warning instead of suggestions.

• A shared stripLocalDomainSuffix helper in packages/common now derives the suggestion: the exact .wp.local suffix (Studio's default) is stripped entirely; otherwise only a single trailing .local label is removed, so mysite.com.local suggests mysite.com.
• Both front ends use it — the agentic UI (which did no stripping) and the classic UI (which only handled exact .wp.local).

Testing Instructions

1. Create a Studio site with the default custom domain (<site-name>.wp.local).
2. In the agentic UI, start the publish flow and follow "Create a new WordPress site" — the domain search should be prefilled with the bare site name and show real suggestions, no warning.
3. Repeat from the classic UI sync flow.
4. Edge cases: a site with custom domain mysite.local prefills mysite; mysite.com.local prefills mysite.com.

Pre-merge Checklist

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

[wpmobilebot]

📊 Performance Test Results

Comparing 8ab0c24 vs trunk

app-size

Metric	trunk	8ab0c24	Diff	Change
App Size (Mac)	1425.05 MB	1425.05 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	8ab0c24	Diff	Change
load	1186 ms	1204 ms	+18 ms	⚪ 0.0%

site-startup

Metric	trunk	8ab0c24	Diff	Change
siteCreation	7515 ms	7551 ms	+36 ms	⚪ 0.0%
siteStartup	3370 ms	3373 ms	+3 ms	⚪ 0.0%

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Results are median values from multiple test runs.

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