refactor(ci): move flake check from CI to separate daily workflow

Move Nix flake input checking from the main CI pipeline to a dedicated
daily scheduled workflow to reduce CI build time while maintaining
regular dependency monitoring.

.github/workflows/ci.yml

@@ -12,17 +12,6 @@ env:
   CARGO_TERM_COLOR: always
 
 jobs:
-  self-care:
-    name: Flake self-check
-    runs-on: ubuntu-latest
-    timeout-minutes: 30
-    steps:
-      - uses: actions/checkout@v4
-      - name: Check Nix flake inputs
-        uses: DeterminateSystems/flake-checker-action@v9
-        with:
-          fail-mode: true
-
   pre-commit-checks:
     name: "Cargo fmt, typos"
     runs-on: ubuntu-latest

.github/workflows/daily-flake-check.yml

@@ -0,0 +1,20 @@
+name: Daily Flake Check
+
+on:
+  schedule:
+    # Run daily at 6 AM UTC
+    - cron: '0 6 * * *'
+  # Allow manual trigger
+  workflow_dispatch:
+
+jobs:
+  flake-check:
+    name: Daily Nix Flake Check
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    steps:
+      - uses: actions/checkout@v4
+      - name: Check Nix flake inputs
+        uses: DeterminateSystems/flake-checker-action@v9
+        with:
+          fail-mode: true