docs(todo): add queued workon.md for grokkit workon command

Added a new todo item in todo/queued/workon.md describing the 'grokkit workon' command, its benefits, and a high-level implementation placeholder.

todo/queued/workon.md

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+# `grokkit workon`
+
+**Description**: selects a todo item out of the todo queue; creates a branch based on the item; generates a plan for implementation.
+
+**Benefits**:
+
+1. Boosts productivity by automating routine tasks.
+
+**High-level implementation**:
+
+1. TBD