Teachers can claim out-of-scope students as theirs (JTP/ayudante takeover)

Elazar directive 2026-07-23 (CLI, during /usuarios unified-route dispatch): teachers need a self-service path to claim a student currently NOT theirs — take over the JTP/ayudante assignment. Deferred out of the /usuarios MVP (MVP scope = teachers edit in-scope students' comisión/JTP/ayudante only). Relates to MARS-108 (teacher-facing reassignment surface).