Partner commanders: Partner, Partner—<variant>, Partner with, Background, Doctor

A Commander/Brawl deck can now have two commanders when their abilities
allow it. Partner abilities are parsed from oracle text (Scryfall's keyword
list files every "Partner—Survivors"-style variant under plain "Partner"),
and a second search box offers only cards that can pair with the commander.
The legality check accepts a valid pair and checks color identity against
both; suggestions and decklist import/export handle both commanders.

Closes #4

crates/mtgc-core/src/model.rs

@@ -214,6 +214,10 @@ pub struct DeckEntry {
     /// card's `image_uri`/`back_image_uri` already reflect it (falling back
     /// to the default when the id no longer resolves).
     pub preferred_scryfall_id: Option<String>,
+    /// The card's partner-style ability, which lets it share the command
+    /// zone with a second commander.
+    #[serde(default)]
+    pub partner: Option<crate::partner::Partner>,
 }
 
 /// One printing of a card, for the printing picker and copy labels.