Email bridge tools via builtin C API

[tnm]

Summary

• port email bridge feature onto the current builtin tool registry architecture in builtin_tools.def
• add firmware bridge client and email tool handlers: email_send, email_list, email_read
• add bridge provisioning support in provision.sh and provision-dev.sh
• add host tests for email tools and provisioning bridge args/CSV wiring

Details

• adds main/bridge_client.c and main/tools_email.c
• registers email tools in built-in registry and handler declarations
• adds bridge_url / bridge_key NVS keys and marks them sensitive
• wires host test runner/build for email tests with bridge/tool-common mocks

Validation

• ./scripts/test.sh host

Supersedes #23.