feat: add --color flag to control ANSI color output

[veeceey]

Summary

Adds a global --color flag with three modes: auto (default), never, and always.

• --color never disables all ANSI color codes in output, useful for CI/CD pipelines and log files
• --color always forces color output even when not connected to a terminal
• --color auto (default) preserves existing behavior - colors when outputting to a terminal

The implementation sets color.NoColor from the fatih/color package, which is used throughout trivy for both log-level colorization and table output severity highlighting.

Also supports config file (color: never) and env var (TRIVY_COLOR=never).

Files changed

• pkg/flag/global_flags.go - Added ColorFlag definition and wired it into GlobalFlagGroup / GlobalOptions
• pkg/commands/app.go - Added applyColorSetting() called in PersistentPreRunE before logger init
• pkg/commands/app_test.go - Tests for applyColorSetting and --color CLI flag validation

Closes #1091

Test plan

• TestApplyColorSetting verifies color.NoColor is set correctly for never and always
• TestColorFlag verifies the flag is accepted with valid values and rejected with invalid values
• All existing tests continue to pass

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