refactor: remove the dead state-to-todo mapping subsystem

`pearl-state-to-todo-mapping` and its derived regex stopped doing real work once keyword rendering moved to slugifying the Linear state name (`pearl--state-name-to-keyword`) and the keyword-to-state direction moved to resolving through the team's workflow states. The defcustom fed only `pearl--get-todo-states-pattern`, which had no caller left, and the two cache vars existed only to serve that dead function. State extraction reads `org-get-todo-state` off the buffer's `#+TODO` line, so nothing live touched the mapping.

I removed the defcustom, both cache vars, and the dead function, and dropped the three pattern tests that exercised it. Removing the defcustom orphaned the mapping bindings that ~12 test setup macros still carried. They were passing through slugification rather than the map, so I stripped them too and kept the `org-todo-keywords` bindings the temp buffers actually need. Also dropped the stale README config row.

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