📝 Docs: email customization guide, migration path, fix the wrong override docs

[PierreBrisorgueil]

What

Fix early (independent, can ship right away):

• The welcome email section of MIGRATIONS.md, which tells downstreams to edit config/templates/welcome.html.
• The myproject.config.js header, which names the wrong filename. The loader reads config/defaults/<project>.config.js.

Land last, after the fragments slice:

• A customization guide for the mailer (e.g. lib/helpers/mailer/README.md) covering:
  • the 3 levels, with a config/defaults/<project>.config.js example
  • the namespace rule: Devkit owns the flat files and _* folders, and downstreams use config/templates/<project>/
  • legacy full-document passthrough
  • that mailer.from accepts "Name" <address> for a sender display name
• A MIGRATIONS.md entry:
  • templates that exist only in a downstream need nothing
  • a shared template edited in place: move it to the project folder, add a map entry, restore the shared file
  • shell changes are expected
• Feature skill (.claude/skills/feature/SKILL.md:101,174,195): keep the flat folder for Devkit module authors, and add that a downstream's new templates go to config/templates/<project>/ with a map entry.

Acceptance criteria

• No doc tells a downstream to edit a shared template.
• The guide's config example matches a config used in the mailer tests.

Depends on: fragments (guide part only).