fix(discover): add cargo nextest to rewrite rule

`rtk cargo nextest` is implemented in src/cmds/rust/cargo_cmd.rs with
dedicated filtered output, but the discover rewrite pattern only matched
`cargo (build|test|clippy|check|fmt|install)`. The Claude Code hook
therefore never rewrote `cargo nextest` calls, so users invoking nextest
directly received raw output instead of RTK's compact summary.

Changes:
- Add `nextest` to the cargo pattern alternation in src/discover/rules.rs
- Register `("nextest", 90.0)` in subcmd_savings (matches `test`, since
  both filters collapse to a one-line pass/fail summary)
- Add classify + rewrite + with-args tests in src/discover/registry.rs
- Update `test_registry_covers_all_cargo_subcommands` to include
  `install` and `nextest` (the loop had drifted out of sync with the
  CargoCommand enum when `Install` was added previously)

Closes #2046

Author: maxmilian

src/discover/registry.rs

@@ -1100,11 +1100,47 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_classify_cargo_nextest() {
+        assert_eq!(
+            classify_command("cargo nextest run"),
+            Classification::Supported {
+                rtk_equivalent: "rtk cargo",
+                category: "Cargo",
+                estimated_savings_pct: 90.0,
+                status: RtkStatus::Existing,
+            }
+        );
+    }
+
+    #[test]
+    fn test_rewrite_cargo_nextest() {
+        assert_eq!(
+            rewrite_command_no_prefixes("cargo nextest run", &[]),
+            Some("rtk cargo nextest run".into())
+        );
+    }
+
+    #[test]
+    fn test_classify_cargo_nextest_with_args() {
+        assert_eq!(
+            classify_command("cargo nextest run --no-fail-fast --release"),
+            Classification::Supported {
+                rtk_equivalent: "rtk cargo",
+                category: "Cargo",
+                estimated_savings_pct: 90.0,
+                status: RtkStatus::Existing,
+            }
+        );
+    }
+
     #[test]
     fn test_registry_covers_all_cargo_subcommands() {
-        // Verify that every CargoCommand variant (Build, Test, Clippy, Check, Fmt)
-        // except Other has a matching pattern in the registry
-        for subcmd in ["build", "test", "clippy", "check", "fmt"] {
+        // Verify that every CargoCommand variant (Build, Test, Clippy, Check, Fmt,
+        // Install, Nextest) except Other has a matching pattern in the registry.
+        for subcmd in [
+            "build", "test", "clippy", "check", "fmt", "install", "nextest",
+        ] {
             let cmd = format!("cargo {subcmd}");
             match classify_command(&cmd) {
                 Classification::Supported { .. } => {}

src/discover/rules.rs

@@ -44,12 +44,12 @@ pub const RULES: &[RtkRule] = &[
         subcmd_status: &[],
     },
     RtkRule {
-        pattern: r"^cargo\s+(build|test|clippy|check|fmt|install)",
+        pattern: r"^cargo\s+(build|test|clippy|check|fmt|install|nextest)",
         rtk_cmd: "rtk cargo",
         rewrite_prefixes: &["cargo"],
         category: "Cargo",
         savings_pct: 80.0,
-        subcmd_savings: &[("test", 90.0), ("check", 80.0)],
+        subcmd_savings: &[("test", 90.0), ("check", 80.0), ("nextest", 90.0)],
         subcmd_status: &[("fmt", RtkStatus::Passthrough)],
     },
     RtkRule {