Give the justfile the rust skill's shape: a default `all` aggregate
recipe over the previous flat default. CI now installs just and runs
`just clippy test` instead of calling cargo directly, so the coverage
and mutants recipes can slot into one place once that testing work
lands. Keeps the existing permissions hardening.

Assisted-by: Claude Opus 4.8 <noreply@anthropic.com>

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d22545d..34722b6 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -17,6 +17,6 @@ jobs:
         with:
           persist-credentials: false
       - run: rustup component add clippy rustfmt
+      - run: cargo install just
       - run: cargo fmt --all --check
-      - run: cargo clippy --workspace -- -D warnings
-      - run: cargo test --workspace
+      - run: just clippy test
diff --git a/justfile b/justfile
index d77df3d..df7fe1d 100644
--- a/justfile
+++ b/justfile
@@ -1,16 +1,18 @@
-default: check fmt clippy test
-
-check:
-    cargo check --workspace
+default: all
 
 fmt:
     cargo fmt --all
 
+check:
+    cargo check --workspace
+
 clippy:
     cargo clippy --workspace -- -D warnings
 
 test:
     cargo test --workspace
 
+all: fmt clippy test
+
 install:
     cargo install --locked --path .