Add a LeetCode problem to a cheatsheet

Turn a problem (or a pattern just learned) into a cheatsheet section in the shape the other ~130 sheets under doc/cheatsheet/ already use — filed under the pattern that owns it, with every snippet run, every moved anchor followed, and the 繁體中文 overlay kept level.

Invocation: /lc-cheatsheet <LC number> [into <sheet-slug>] [as <mode>]

/lc-cheatsheet 1650 into binary_tree as variation
/lc-cheatsheet 239 into sliding_window          # mode inferred -> example
/lc-cheatsheet 496 as pattern                   # sheet inferred from the technique
/lc-cheatsheet monotonic_queue as sheet         # a whole new file
add LC 1650 to the LCA part of binary_tree.md   # the plain-English form works too