rename: mail-mcp -> Carrier

Sulkta naming convention. Carrier (the carrier pigeon — single-
purpose, reliable, comes back every time) sits alongside Aldabra
(giant tortoise), Hawk (eBay-resale eyes), Skald (Norse storyteller),
cWHO (monitoring), Cauldron (meal planning), Clawdforge (build), tny
(URL shortener), ktra (cargo registry).

The full audit/cleanup/Phase-A-B-C arc happened under the mail-mcp
name; this commit just renames the identity:

- Cargo workspace + crate package name: mail-mcp -> carrier
- Binary name: mail-mcp -> carrier
- USER_AGENT header: mail-mcp/<ver> -> carrier/<ver>
- Config env var: MAIL_MCP_CONFIG -> CARRIER_CONFIG
- Default config path: ~/.config/mail-mcp/config.toml -> ~/.config/carrier/config.toml
- ServerHandler instructions reference: 'mail-mcp' -> 'Carrier'
- README + repo URL refs updated
- Workspace path: /root/build/mail-mcp -> /root/build/carrier
- Git remote: gitea:Sulkta-Coop/mail-mcp -> gitea:Sulkta-Coop/carrier

Tool names stay mail_* (mail_send, mail_inbox_list, mail_reply, etc.)
because they describe the email domain — same convention as Aldabra
keeps wallet_*/chain_*/dao_*/escrow_*. The server-level identity is
Carrier; the tools it carries are mail.

config.example.toml

@@ -1,4 +1,4 @@
-# mail-mcp config — copy to ~/.config/mail-mcp/config.toml, chmod 600.
+# Carrier config — copy to ~/.config/carrier/config.toml, chmod 600.
 #
 # Passwords are NEVER inline. Each account names an env var (`password_env`)
 # AND a fallback file (`password_file`). Lookup order: