docs/adapters/registry.md: Fossil misclassified as VCS adapter; Gmail omitted from mail-source

[rbowen]

What should happen

1. Reclassify Fossil from "VCS adapter" to "forge adapter" (it's under tools/fossil/ alongside Bitbucket and SourceHut, not alongside tools/vcs/).
2. Add Gmail to the mail-source adapter list (it's implemented at tools/gmail/).

Why

The registry is the reference for contributors deciding where to add new adapter support. Misclassification sends them to the wrong pattern to follow.

Which layer

Documentation (docs/, README.md, CONTRIBUTING.md)