feat(email): add email bridge tools with host coverage

[tnm]

Summary

• add bridge-backed email tools and supporting firmware integration
• add provisioning and docs updates for email bridge configuration
• add extensive host tests and mocks for email tool behavior

Scope

• intentionally includes only email feature/test work split from other branches
• excludes unrelated diagnostics formatting changes

Testing

• ./scripts/test.sh host

[tnm]

Closing as superseded by a fresh email bridge/tools reimplementation on top of current main (builtin_tools.def registry path).