Add command target presets for extras init

[TIR44]

Addresses #146.

Summary

• Added command target preset expansion for extras init commands (claude / claude-code, cursor, codex).
• Mapped global presets to ~/.claude/commands, ~/.cursor/commands, and ~/.codex/prompts; mapped project presets to .claude/commands and .cursor/commands.
• Rejected the project codex preset with a clear message because Codex prompt files are home-scoped rather than project-discovered.
• Updated CLI help, TUI placeholders, and docs for command invocation differences.

Validation

• go test ./cmd/skillshare -run 'TestResolveExtraInitTargetPresets|TestCmdExtrasInit_Commands' -count=1\n- make test-unit

[gemini-code-assist]

Code Review

This pull request introduces command target presets (claude, cursor, and codex) for the 'extras init commands' command, allowing users to easily configure common agent directories in both global and project modes. It updates the command-line help, documentation, and interactive TUI placeholders, and adds corresponding unit tests. The review feedback highlights a potential compatibility issue in the tests where t.Chdir is used, which requires Go 1.24 or later, and suggests a backward-compatible alternative using os.Chdir and t.Cleanup.

[gemini-code-assist]

Using t.Chdir requires Go 1.24 or later. If the project supports older Go versions (such as Go 1.22 or 1.23), this will cause a compilation error. Consider using os.Chdir with t.Cleanup to restore the working directory for backward compatibility. Note that you will also need to import the os package.

Suggested change

	t.Chdir(root)
	oldWD, err := os.Getwd()
	if err != nil {
	t.Fatalf("get wd: %v", err)
	}
	if err := os.Chdir(root); err != nil {
	t.Fatalf("chdir: %v", err)
	}
	t.Cleanup(func() {
	_ = os.Chdir(oldWD)
	})