chore: upgrade rmcp 1.8.0 -> 3.1.2

Compile-clean upgrade verified: zero source changes needed, full test
suite green, clippy clean. Coyote's rmcp API surface (14 items) dodges
all 2.0/3.0 breaking changes; the "Auth required" error string matched
by is_auth_required_error is intact in 3.1.2.

Cargo.toml

@@ -84,7 +84,7 @@ duct = "1.0.0"
 argc = "1.23.0"
 strum_macros = "0.27.2"
 indoc = "2.0.6"
-rmcp = { version = "1.5.0", features = [
+rmcp = { version = "3.1.2", features = [
   "client",
   "transport-child-process",
   "transport-streamable-http-client-reqwest",
@@ -150,4 +150,4 @@ path = "src/main.rs"
 [profile.release]
 lto = true
 strip = true
-opt-level = "z"
+opt-level = "z"