refactor with using flagrun and saferio

[kazeburo]

PR Type

Enhancement, Bug fix, Tests, Documentation

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Description

• Adopt flagrun for standardized plugin execution.

• Use saferio for secure atomic state handling.

• Reject state-file symlinks without target modification.

• Expand Japanese usage and metric documentation.

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Diagram Walkthrough

flowchart LR
  Runtime["Plugin runtime"] 
  Flagrun["flagrun command runner"]
  Metrics["Mackerel metrics plugin"]
  State["CPU state JSON"]
  Saferio["saferio secure I/O"]
  Runtime -- "delegates execution" --> Flagrun
  Flagrun -- "runs" --> Metrics
  Metrics -- "reads and writes" --> State
  State -- "secured by" --> Saferio

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File Walkthrough

	Relevant files
Enhancement	2 files main.go Replace custom CLI flow with flagrun                                          +8/-44    usageplugin.go Move plugin methods onto runtime options                                  +14/-17
Miscellaneous	2 files open_unix.go Remove Unix-specific secure file opener                                    +0/-15    open_windows.go Remove Windows-specific secure file opener                              +0/-12
Bug fix	1 files statsfile.go Delegate JSON state I/O to saferio                                              +3/-40
Tests	2 files statsfile_test.go Test legacy state and symlink safety                                          +46/-19  usageplugin_test.go Update metric tests for option receiver                                    +7/-7
Configuration changes	1 files Makefile Build all Go sources without commit metadata                          +4/-5
Documentation	1 files README.md Expand Japanese installation and metric documentation        +128/-30
Dependencies	1 files go.mod Add flagrun and saferio dependencies                                          +4/-1

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

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⏱️ Estimated effort to review: 2 🔵🔵⚪⚪⚪

🧪 PR contains tests

🔒 No security concerns identified

⚡ No major issues detected

[github-actions]

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