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: mail-mcp -> carrier
- Git remote: mail-mcp -> 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.

crates/carrier/Cargo.toml

@@ -1,17 +1,17 @@
-# mail-mcp — the binary. Stdio MCP server exposing SMTP send + IMAP
+# Carrier — the binary. Stdio MCP server exposing SMTP send + IMAP
 # read tools. Spawned per-session by any MCP client.
 
 [package]
-name = "mail-mcp"
+name = "carrier"
 version.workspace = true
 edition.workspace = true
 license.workspace = true
 repository.workspace = true
 authors.workspace = true
-description = "Rust MCP server for email (SMTP send + IMAP read)"
+description = "Carrier — Rust MCP server for email (SMTP send + IMAP read)"
 
 [[bin]]
-name = "mail-mcp"
+name = "carrier"
 path = "src/main.rs"
 
 [dependencies]

crates/carrier/src/config.rs

@@ -1,6 +1,6 @@
 //! TOML config + password resolution.
 //!
-//! Config path: `$MAIL_MCP_CONFIG` env, or `~/.config/mail-mcp/config.toml`.
+//! Config path: `$CARRIER_CONFIG` env, or `~/.config/carrier/config.toml`.
 //!
 //! Password lookup per account:
 //!   1. env var named by `password_env`
@@ -160,12 +160,12 @@ fn check_chmod(_path: &std::path::Path) -> Result<()> {
 }
 
 fn config_path() -> Result<PathBuf> {
-    if let Ok(p) = std::env::var("MAIL_MCP_CONFIG") {
+    if let Ok(p) = std::env::var("CARRIER_CONFIG") {
         return Ok(PathBuf::from(shellexpand::tilde(&p).into_owned()));
     }
     let home = dirs::config_dir()
         .ok_or_else(|| anyhow!("could not resolve $XDG_CONFIG_HOME / ~/.config"))?;
-    Ok(home.join("mail-mcp").join("config.toml"))
+    Ok(home.join("carrier").join("config.toml"))
 }
 
 #[cfg(test)]

crates/carrier/src/main.rs

@@ -1,8 +1,14 @@
-//! mail-mcp — MCP server entry point.
+//! Carrier — a Rust MCP email server.
 //!
 //! Speaks MCP over stdio. Any MCP client launches this as a
 //! subprocess
-//! and gets `mail_send` + `mail_inbox_list` + `mail_inbox_read`.
+//! and gets the 10-tool mail surface (mail_send / mail_inbox_list /
+//! mail_inbox_read / mail_folder_list / mail_search / mail_thread /
+//! mail_move / mail_mark / mail_attachment_get / mail_reply).
+//!
+//! Named after the carrier pigeon — single-purpose, reliable, comes
+//! back every time. Tool names stay `mail_*` because they describe
+//! the domain; only the server identity is Carrier.
 //!
 //! Logging is stderr-only — stdout belongs to the JSON-RPC transport.
 
@@ -50,7 +56,7 @@ async fn run() -> Result<()> {
     tracing::info!(
         accounts = cfg.accounts.len(),
         default_account = %cfg.default_account,
-        "mail-mcp starting"
+        "carrier starting"
     );
 
     let service = MailService::new(cfg);

crates/carrier/src/smtp.rs

@@ -5,7 +5,7 @@
 //!   - `Message-ID`   — `<uuid-v4@{message_id_domain}>` — own-domain, not local hostname
 //!   - `From`         — `name <addr>`
 //!   - `MIME-Version` — lettre auto
-//!   - `User-Agent`   — `mail-mcp/<crate version>`
+//!   - `User-Agent`   — `carrier/<crate version>`
 //!   - `In-Reply-To`  — if provided
 //!   - `References`   — if provided (space-joined)
 //!
@@ -52,7 +52,7 @@ pub struct SendOutput {
     pub sent_at: String, // RFC-3339
 }
 
-const USER_AGENT: &str = concat!("mail-mcp/", env!("CARGO_PKG_VERSION"));
+const USER_AGENT: &str = concat!("carrier/", env!("CARGO_PKG_VERSION"));
 
 /// Hard caps to keep an MCP-driver-gone-wrong from OOM-ing the box.
 /// Match Gmail's effective 25 MB per-message ceiling — any single attachment

crates/carrier/src/tools.rs

@@ -601,7 +601,7 @@ impl ServerHandler for MailService {
         ServerInfo {
             capabilities: ServerCapabilities::builder().enable_tools().build(),
             instructions: Some(
-                "mail-mcp — Rust MCP server for Sulkta-hosted email. Tools: \
+                "Carrier — a Rust MCP email server. Tools: \
                  mail_send, mail_inbox_list, mail_inbox_read, mail_folder_list, \
                  mail_search, mail_thread, mail_move, mail_mark, \
                  mail_attachment_get, mail_reply. Default account from config; \