justfile(fix[watch]): Fix variable expansion in watch tasks

why: Watch tasks failed with "unbound variable" error because ${{ var }}
expanded to $find which bash -u treats as undefined variable.
what:
- Remove $ prefix from {{ var }} in watch-test, watch-ruff, watch-mypy
- Fix mypy task: $(${{ py_files }}) → $({{ py_files }})

Author: tony

justfile

@@ -28,7 +28,7 @@ watch-test:
     #!/usr/bin/env bash
     set -euo pipefail
     if command -v entr > /dev/null; then
-        ${{ py_files }} | entr -c just test
+        {{ py_files }} | entr -c just test
     else
         just test
         just _entr-warn
@@ -65,7 +65,7 @@ watch-ruff:
     #!/usr/bin/env bash
     set -euo pipefail
     if command -v entr > /dev/null; then
-        ${{ py_files }} | entr -c just ruff
+        {{ py_files }} | entr -c just ruff
     else
         just ruff
         just _entr-warn
@@ -74,15 +74,15 @@ watch-ruff:
 # Run mypy type checker
 [group: 'lint']
 mypy:
-    uv run mypy $(${{ py_files }})
+    uv run mypy $({{ py_files }})
 
 # Watch files and run mypy on change
 [group: 'lint']
 watch-mypy:
     #!/usr/bin/env bash
     set -euo pipefail
     if command -v entr > /dev/null; then
-        ${{ py_files }} | entr -c just mypy
+        {{ py_files }} | entr -c just mypy
     else
         just mypy
         just _entr-warn