feat: add Usage option for custom help message in Flagrun

[kazeburo]

PR Type

Enhancement, Tests, Documentation

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Description

• Add configurable parser usage text option

• Preserve required-arguments usage as default

• Test custom usage in help output

• Document Usage configuration behavior

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Diagram Walkthrough

flowchart LR
  Config["flagrun.Usage option"]
  Parser["Go flags parser"]
  Help["Custom help usage output"]
  Config -- "sets `Usage`" --> Parser
  Parser -- "renders" --> Help

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File Walkthrough

	Relevant files
Enhancement	flagrun.go Add configurable parser usage text                                              flagrun.go Add Usage field to Flagrun. Add Usage(string) functional option. Apply custom usage to the parser when provided. Retain required-arguments usage only without custom text. +11/-1
Tests	flagrun_go_test.go Test custom usage help output                                                        flagrun_go_test.go Add help-output test for Usage. Verify custom usage is written to stdout. Confirm help exits with OK. +16/-0
Documentation	README.md Document configurable usage option behavior                            README.md Document the flagrun.Usage option. Explain its default and ArgsRequired interaction. +1/-0

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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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🧪 PR contains tests

🔒 No security concerns identified

⚡ No major issues detected

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