Refactor : improve TestDeleteContext robustness and isolation using temporary directories

まだ誰も着手していません。

### Reason/Context

Currently, `TestDeleteContext` in `cmd/context_test.go` relies on a **hardcoded** file path
`/testdata/local.config`

This approach causes the test to fail on filesystems that do not fully
support Unix-style permissions (such as NTFS or FAT32 partitions commonly used on external drives or
Windows-mounted volumes), as the test attempts to verify restrictive file permissions (0600).

### Description

The enhancement involves refactoring the `TestDeleteContext` function. Instead of using a static file
path, the test will now:

1. Utilize `t.TempDir()` to create a unique, isolated directory for each test run.
2. Construct the configuration file path dynamically using f`ilepath.Join()`
3. Clean up automatically after the test finishes, as `t.TempDir()` is managed by the Go testing
package.

### Implementation ideas

_No response_

Open cmd/context_test.go and start with TestDeleteContext. Run that test to observe its current behavior, then verify that the test uses t.TempDir() and filepath.Join() instead of the hardcoded path. Done means the test remains isolated, cleans up automatically, and passes on its supported filesystems.

索引モデルが issue の本文から書いたものです。