feat: Add support for reading command-line options from dotfiles (#191)

[adminlip]

Summary

Implements #191: CLI applications can now read default option values from .<command> dotfiles in the current working directory, its parent directories, and the user home directory.

Design

• FileOptions service trait with Live (JVM/Native) and Noop (JS/tests) implementations
• Priority order: closer files win (cwd > parents > home), CLI args always override file defaults
• OrElse root guard: when the top-level command is (a | b), file-options lookup is silently skipped (no unique name to anchor on)
• Platform shims: FileOptionsPlatformSpecific provides default = Live on JVM/Native, default = Noop on Scala.js

Changes

New files

• FileOptions.scala — service trait + Live/Noop implementations
• FileOptionsPlatformSpecific.scala (js/jvm/native) — platform-specific default value
• FileOptionsOverrideSpec.scala — tests for priority/override semantics
• LiveFileOptionsSpecShared.scala — filesystem tests using TestSystem

Modified files

• CliApp.scala — new runWithFileArgs/runWithoutFileArgs methods; run now uses platform default
• Command.scala — parse methods accept fromFiles: List[FileOptions.OptionsFromFile] parameter
• Options.scala — validate merges file-derived options before CLI args; new mergeFileOptions/mergeFileAndInputOptions helpers

How it works

1. CliApp.runWithFileArgs extracts the root command name (e.g. git)
2. FileOptions.getOptionsFromFiles("git") searches for .git files in cwd → parents → home
3. Each file is parsed as CLI args, merged by priority (closer wins)
4. Options.validate merges file defaults with explicit CLI args (CLI wins)
5. The merged options are validated and the result is returned

Backward compatibility

• Command.parse gets fromFiles: List[FileOptions.OptionsFromFile] = Nil — existing call sites unchanged
• CliApp.run now consults dotfiles by default (via FileOptions.default), but apps that don't have dotfiles see no difference
• CliApp.runWithoutFileArgs provides an explicit opt-out

/claim #191

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