fix: stabilize flaky command tests and get suite passing

[kusutori]

Motivation

• Recent test rewrites caused multiple flaky failures due to shared Cobra flag state and brittle assertions against colored/formatted help output.
• Tests also failed to build because of an unused import in internal/download/download_test.go.

Description

• Reset Cobra flags recursively before each command test case and restore SilenceUsage/SilenceErrors after running to avoid cross-test state leakage (cmd/command_exec_test.go).
• Improve output capture by piping os.Stdout/os.Stderr in the test harness while preserving RootCmd.SetOut/SetErr buffers (cmd/command_exec_test.go).
• Add wantErrText to the test harness and update add, remove, and completion tests to assert canonical error text from returned error instead of relying on colored/help-formatted output (cmd/add_cmd_test.go, cmd/remove_cmd_test.go, cmd/completion_cmd_test.go).
• Remove an unused strings import that caused a test build failure (internal/download/download_test.go).

Testing

• Ran go test ./... and the entire test suite passed successfully.
• Ran go test ./... -cover to confirm coverage reporting and it completed successfully.

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[kusutori]

@copilot fix the CI failed