Templates tab persistence

Part of #836. Wires the Templates tab. Depends on the Color/foundation PR. Final per-tab PR if scheduled last; can also land before others — order among PRs 2–5 is flexible.

Scope

• TemplatePartsPanel → top-level templateParts array.
• CustomTemplatesPanel → top-level customTemplates array (each entry: { name, title, postTypes: [...] }).
• Update readme.txt to document the Edit Theme Settings feature (if this PR lands last; otherwise the readme update goes with whichever per-tab PR completes the feature).

Acceptance criteria

• Adding / removing / editing template parts persists.
• Adding a custom template with multiple post types persists with the array intact.
• Removing a template removes the entry only (does not touch template files on disk — that's a separate concern).
• If this is the last per-tab PR: readme.txt describes the feature in the Description and Changelog sections.

Out of scope

• Touching template files on disk when entries are removed.
• WP-CLI.

References

• Tracking issue: #836

Trace TemplatePartsPanel and CustomTemplatesPanel to find the persistence path and how the top-level templateParts and customTemplates arrays are represented. Verify adding, editing, and removing entries, including multiple postTypes and entry-only removal. If this is the last per-tab PR, update readme.txt Description and Changelog sections.