Track explicitly set config options to enable overriding machine-wide defaults

[Copilot]

Config options at default values were not persisted to disk. When machine-wide config sets non-default values, users cannot override back to defaults since user config files omit default-valued options.

Changes

• Added _explicitlySetOptions HashSet to track options set via SetOption() or loaded from config files
• Modified save logic in ToIniData() to persist tracked options even when at default values
• Updated ResetOption() to remove options from tracking, allowing inheritance from machine-wide config

Usage

# Override machine-wide network_use=minimal back to default
0install config network_use full

# Remove user override, inherit from machine-wide
0install config network_use default

The tracking mechanism distinguishes between:

• Unset options (inherit from machine-wide)
• Explicitly set to default (override machine-wide)

Added tests covering machine-wide override scenarios, persistence across save/load cycles, and reset behavior.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Allow overriding machine-wide config options back to default</issue_title>
<issue_description>Currently, when a config option is set to it's default value, the value is not persisted on disk. This works fine, unless a non-default value is set in the machine-wide config. Find a way to solve this.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Allow overriding machine-wide config options back to default #191

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