feat(payload-helper): Add reusable Slug field helper

[ainsleyclark]

Motivation

• Provide a reusable SlugField helper to standardise slug generation and admin UI across collections and reduce duplicated slug logic.

Description

• Add a new Slug field helper under packages/payload-helper/src/fields/Slug that returns a [TextField, CheckboxField] pair and supports overrides via slugOverrides and checkboxOverrides.
• Add formatSlug and formatSlugHook in formatSlug.ts to automatically format fallback values into URL-friendly slugs during beforeValidate.
• Add a lockable React admin component Component.tsx and stylesheet index.scss to provide a lock/unlock UI for auto-generated slugs, and wire the component via the field admin.components.Field client path.
• Export SlugField and related types from packages/payload-helper/src/fields/index.ts and re-export from the package root src/index.ts, and add a Changeset (.changeset/fresh-turkeys-bathe.md) marking a minor feature bump for @ainsleydev/payload-helper.

Testing

• Attempted to run formatting via pnpm --filter @ainsleydev/payload-helper format, but this failed due to an environment package-manager bootstrap/network tunnel error (HTTP 403), so formatting/type-build steps were not completed.
• No automated unit tests were executed in this environment after the change; the changes were committed successfully.

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Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.26%. Comparing base (7f6b060) to head (a2a8b19).
⚠️ Report is 508 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #424      +/-   ##
==========================================
+ Coverage   64.59%   70.26%   +5.67%     
==========================================
  Files         154      187      +33     
  Lines        6064     7439    +1375     
==========================================
+ Hits         3917     5227    +1310     
+ Misses       2064     2012      -52     
- Partials       83      200     +117     

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