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.

Cargo.toml

@@ -1,18 +1,22 @@
-# Cargo workspace root for mail-mcp.
+# Cargo workspace root for Carrier — Sulkta's Rust MCP email server.
 #
-# One crate today (mail-mcp), workspace shape so we can grow without
+# Named after the carrier pigeon: single-purpose, reliable, comes back
+# every time. Carries Sulkta-hosted mail (kayos@/cobb@/abby@/bay@/jay@)
+# to and from any MCP client.
+#
+# One crate today (carrier), workspace shape so we can grow without
 # rework. Same pattern as aldabra.
 #
 # Workspace deps pinned here; each crate references with `foo = { workspace = true }`.
 [workspace]
 resolver = "2"
-members = ["crates/mail-mcp"]
+members = ["crates/carrier"]
 
 [workspace.package]
 version = "0.1.0"
 edition = "2021"
 license = "MIT"
-repository = "http://192.168.0.5:3001/Sulkta-Coop/mail-mcp"
+repository = "http://192.168.0.5:3001/Sulkta-Coop/carrier"
 authors = ["Cobb <cobb@sulkta.com>", "Kayos <kayos@sulkta.com>"]
 
 [workspace.dependencies]
@@ -78,7 +82,7 @@ chrono = { version = "0.4", default-features = false, features = ["clock"] }
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 
-# Dirs lookup for `~/.config/mail-mcp/config.toml` default path
+# Dirs lookup for `~/.config/carrier/config.toml` default path
 dirs = "5"
 
 # Shell-style env-var expansion for the `password_file` setting