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.

Cargo.lock

@@ -137,6 +137,34 @@ version = "1.11.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1e748733b7cbc798e1434b6ac524f0c1ff2ab456fe201501e6497c8417a4fc33"
 
+[[package]]
+name = "carrier"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "async-imap",
+ "base64 0.22.1",
+ "chrono",
+ "dirs 5.0.1",
+ "futures",
+ "lettre",
+ "mail-parser",
+ "rmcp",
+ "rustls",
+ "rustls-pki-types",
+ "schemars",
+ "serde",
+ "serde_json",
+ "shellexpand",
+ "tokio",
+ "tokio-rustls",
+ "toml",
+ "tracing",
+ "tracing-subscriber",
+ "uuid",
+ "webpki-roots 0.26.11",
+]
+
 [[package]]
 name = "cc"
 version = "1.2.62"
@@ -763,34 +791,6 @@ version = "0.4.29"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5e5032e24019045c762d3c0f28f5b6b8bbf38563a65908389bf7978758920897"
 
-[[package]]
-name = "mail-mcp"
-version = "0.1.0"
-dependencies = [
- "anyhow",
- "async-imap",
- "base64 0.22.1",
- "chrono",
- "dirs 5.0.1",
- "futures",
- "lettre",
- "mail-parser",
- "rmcp",
- "rustls",
- "rustls-pki-types",
- "schemars",
- "serde",
- "serde_json",
- "shellexpand",
- "tokio",
- "tokio-rustls",
- "toml",
- "tracing",
- "tracing-subscriber",
- "uuid",
- "webpki-roots 0.26.11",
-]
-
 [[package]]
 name = "mail-parser"
 version = "0.9.4"