refactor(dev): drop the dev-performance skill, keep its findings at the code

A whole skill was too much for what this is. The parts that are load-bearing —
why the two lookalike cache flags are opposite decisions, the measured numbers,
the corruption remedy, and the benchmarking trap — now live in the config and
script they describe, where someone changing the flag actually reads them.

The trap is the piece worth keeping: `next dev` compiles on demand so startup
time is meaningless, and stopping the server with `kill -9` makes Turbopack
discard a partially-written cache and rebuild silently — which reads as 'the
cache does nothing' and is how this flag stayed wrong for a month.

Dropped rather than relocated: generic advice that was not specific to this repo
(antivirus, Docker-on-macOS, orphaned processes) and a measured no-op
(`optimizePackageImports` for lucide-react changed nothing, 31.6s vs 31.7s).

Author: waleedlatif1