feat(commandline): fluent Command builder, env var fallback, and help section grouping

[deer]

New: Command fluent builder

• Command.create("spin").option(...).section(...).envVar(...).build()
• Deferred registration — all add() calls applied at build() time
• NoSuchMethodException wrapped as IllegalArgumentException

New: env var fallback on CommandLineParser

• registerEnvVar(String envVarName, String prefix) maps an env var to a CLI prefix
• setEnvironmentVariableResolver(Function) for custom resolution (defaults to System::getenv, pass null to disable)
• Env var args are prepended so command-line arguments always take precedence

New: help section grouping on CommandLineParser

• setHelpSection(String name) / Command.section(String name) group options under named headings
• Mirrors cliffy's .group() API — registration-order-based, not annotation-based
• Purely a rendering concern; parse behaviour is unaffected

Tests

• Split CommandLineParserTests into four focused files: CommandLineParserTests, CommandTests, CommandLineParserSectionTests, CommandLineParserEnvVarTests