feat: custom commands (#133)

* Implement custom commands

* Add User: prefix

* Reuse var

* Check if the agent is busy and if so report a warning

* Update README

* fix typo

* Implement user and project scoped custom commands

* Allow for $ARGUMENTS

* UI tweaks

* Update internal/tui/components/dialog/arguments.go

Co-authored-by: Kujtim Hoxha <kujtimii.h@gmail.com>

* Also search in $HOME/.opencode/commands

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Co-authored-by: Kujtim Hoxha <kujtimii.h@gmail.com>