Support --local flag for project-scoped age gating

[SmileyChris]

Add a --local flag (e.g. gestate set 3 --local) that sets the minimum release age only for the current project rather than globally.

The config file should live in the project root, and project-scoped settings should override the global ones. This lets teams enforce a cooldown policy for their project without affecting the developer's global setup.

[SmileyChris]

Also worth considering: when displaying the current age-gating configuration (e.g. gestate show or the interactive status view), it should reflect any local project settings that override the global ones, so the user can see at a glance what's actually in effect.