refactor: rename 7-tentacles → frontier-practices, sideline dialects/me

Resolve the long-standing name collision between two directories both
called "7-tentacles":

- nextgen-languages/7-tentacles/ (separate submodule) is the curriculum
  source of truth and has been renamed to hyperpolymath/tentacles-agentic-syllabus
  and pulled out to the top level.
- my-lang/7-tentacles/ (this subdirectory, inside my-lang) was the
  my-lang-specific applied learner & teacher materials layer — the
  parallel of ~/Desktop/Frontier_Programming_Practices_AffineScript/.
  It has been renamed to my-lang/frontier-practices/ to match that
  pattern and resolve the collision.

Also sideline dialects/me/ to _exploratory/me-scaffolding/ with a
SIDELINED.adoc explaining the architectural pivot. That directory was
an earlier exploratory attempt to build Me as a static standalone
dialect compiler (~2,619 LOC, 12 Rust crates). The real Me is an
on-the-fly agent-generated projection over Solo ⊂ Duet ⊂ Ensemble
built from an A2ML task spec by the aspect-agents in
tentacles-agentic-syllabus/me/. The scaffolding is kept for salvage
value (the 1,388-LOC parser may be mineable later) but not as an
active build target.

Adds a prominent IMPORTANT callout at the top of
frontier-practices/README.adoc explaining the rename and the updated
mental model:
- audience broadened from ages 8-18 to everyone up to senior professors
- My-Lang real dialects are three nested (Solo ⊂ Duet ⊂ Ensemble),
  not four parallel; Me is not a fourth dialect but a projector
- the "7" in "7 cephalopod mascots" is no longer load-bearing

Full rewrites of the README and internal references inside
frontier-practices/ are follow-up tasks.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath