Remove jetpack/subscriptions newsletter signup recommendation

[youknowriad]

Related issues

• Related to: agent-generated newsletter signup blocks rendering empty on Studio sites

How AI was used in this PR

Claude Code edited the agent's plugin-recommendations skill and the related routing references to drop the jetpack/subscriptions block recommendation. I reviewed the diff and confirmed the remaining Jetpack Forms section is unchanged.

Proposed Changes

• Remove the ## Newsletter Signup section from apps/cli/ai/skills/plugin-recommendations/SKILL.md, along with the cross-reference paragraph in the ## Jetpack Forms section and the "newsletter signup" mention in the skill intro/precedence note.
• Drop jetpack/subscriptions from the core/html form-markup policy message in apps/cli/ai/block-content-policy.ts.
• Drop "newsletter signup" from the routing pointers in apps/cli/ai/system-prompt.ts and apps/cli/ai/skills/block-content/SKILL.md.
• Update the stale newsletter/subscriptions guard in apps/cli/ai/tests/system-prompt.test.ts to an equivalent guard against the still-present Jetpack Forms section content (the test verifies on-demand skill content isn't eagerly loaded into the system prompt).

Why

jetpack/subscriptions requires a Jetpack / WordPress.com connection to actually create subscribers. Local Studio sites don't have that connection by default, so when the agent generated newsletter signup markup, the block rendered as an empty form on the frontend. Removing the recommendation prevents that failure mode until we have a story for the connection step. Jetpack Forms (contact form) does not have the same requirement and stays in the skill.

Testing Instructions

• npm test -- apps/cli/ai/tests/block-content-policy.test.ts apps/cli/ai/tests/system-prompt.test.ts — 11/11 pass.
• Ask the agent for a "newsletter signup" or "email subscribe form" on a local site and confirm it no longer reaches for jetpack/subscriptions. The Jetpack Forms / contact-form path should still work for contact forms.

Pre-merge Checklist

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

[wpmobilebot]

📊 Performance Test Results

Comparing ebe4158 vs trunk

app-size

Metric	trunk	ebe4158	Diff	Change
App Size (Mac)	1340.06 MB	1340.06 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	ebe4158	Diff	Change
load	1747 ms	1762 ms	+15 ms	⚪ 0.0%

site-startup

Metric	trunk	ebe4158	Diff	Change
siteCreation	9590 ms	9588 ms	2 ms	⚪ 0.0%
siteStartup	4924 ms	4918 ms	6 ms	⚪ 0.0%

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Results are median values from multiple test runs.

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