Added welcomeEmailsDesignCustomization feature flag

[cmraible]

closes https://linear.app/ghost/issue/NY-1106/create-feature-flag-for-email-settings-customization

Summary

• Adds a new welcomeEmailDesignCustomization private feature flag to enable toggling welcome email design customization on/off
• Adds the flag to the labs settings UI under private features
• Updates the config API snapshot to include the new flag

Test plan

• Unit tests pass (ghost/core/test/unit/shared/labs.test.js — 10 passing)
• Config API e2e snapshot updated and passing
• Toggle visible in Labs → Private features when developer experiments is enabled

[coderabbitai]

Walkthrough

Adds a new private lab feature titled "Welcome Emails Design Customization" with flag welcomeEmailsDesignCustomization. The flag is registered in the private features list and a corresponding UI entry was added to the admin settings to allow toggling. No other behavioral changes or public API signature changes were made.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title accurately and directly summarizes the main change: adding a new feature flag named welcomeEmailsDesignCustomization. It is concise, clear, and captures the primary objective of the changeset.
Description check	✅ Passed	The pull request description clearly relates to the changeset, detailing the addition of a new feature flag for welcome email design customization and its integration into the labs settings UI.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch chris-ny-1106-create-feature-flag-for-email-settings-customization

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[coderabbitai]

Actionable comments posted: 1

🧹 Nitpick comments (1)

.claude/skills/add-private-feature-flag/SKILL.md (1)

19-23: Add an explicit manual UI verification step for Labs visibility.

The test steps currently cover unit/snapshot checks only. Add a short manual check for “Developer Experiments enabled → Labs → Private features toggle visible” to close the loop on UI behavior.

Suggested addition

 3. **Run tests and update the config API snapshot**
    - Unit: `cd ghost/core && yarn test:single test/unit/shared/labs.test.js`
    - Update snapshot and run e2e: `cd ghost/core && UPDATE_SNAPSHOTS=1 yarn test:single test/e2e-api/admin/config.test.js`
    - Review the diff of `ghost/core/test/e2e-api/admin/__snapshots__/config.test.js.snap` to confirm only your new flag was added.
+   - Manually verify in Admin: enable Developer Experiments, then confirm the new toggle appears under **Labs → Private features**.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In @.claude/skills/add-private-feature-flag/SKILL.md around lines 19 - 23,
Update the test steps in the "Run tests and update the config API snapshot"
block to include an explicit manual UI verification: after running the unit and
snapshot commands (the lines referencing ghost/core && yarn test:single and
ghost/core/test/e2e-api/admin/config.test.js), add a short manual check
instructing the tester to enable "Developer Experiments" in the UI, navigate to
Labs, and confirm the "Private features" toggle is visible; reference the
snapshot file ghost/core/test/e2e-api/admin/__snapshots__/config.test.js.snap so
reviewers know to verify only the new flag was added.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In @.claude/skills/add-private-feature-flag/SKILL.md:
- Line 27: The example flag string in SKILL.md is incorrect: replace the
camelCase example `welcomeEmailDesignCustomization` with the canonical key
`welcomeEmailsDesignCustomization` so documentation matches the PR standard and
prevents incorrect flag registration; update the example line that currently
reads "Flags are camelCase strings (e.g. `welcomeEmailDesignCustomization`)." to
use `welcomeEmailsDesignCustomization`.

---

Nitpick comments:
In @.claude/skills/add-private-feature-flag/SKILL.md:
- Around line 19-23: Update the test steps in the "Run tests and update the
config API snapshot" block to include an explicit manual UI verification: after
running the unit and snapshot commands (the lines referencing ghost/core && yarn
test:single and ghost/core/test/e2e-api/admin/config.test.js), add a short
manual check instructing the tester to enable "Developer Experiments" in the UI,
navigate to Labs, and confirm the "Private features" toggle is visible;
reference the snapshot file
ghost/core/test/e2e-api/admin/__snapshots__/config.test.js.snap so reviewers
know to verify only the new flag was added.

---

ℹ️ Review info

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between e92e2bf and 57d38af.

⛔ Files ignored due to path filters (1)

• ghost/core/test/e2e-api/admin/__snapshots__/config.test.js.snap is excluded by !**/*.snap

📒 Files selected for processing (3)

• .claude/skills/add-private-feature-flag/SKILL.md
• apps/admin-x-settings/src/components/settings/advanced/labs/private-features.tsx
• ghost/core/core/shared/labs.js

🚧 Files skipped from review as they are similar to previous changes (2)

• apps/admin-x-settings/src/components/settings/advanced/labs/private-features.tsx
• ghost/core/core/shared/labs.js