new: dev: Add development guidance for AI coding agents under .agents/skills/

This adds a set of skill documents that give AI coding agents the
project's established practices up front instead of having them
rediscovered (or gotten wrong) in every session: the canonical build
and test invocations, the memory-allocator contract, the disciplines
for RCU mutation, per-loop sharded structures, struct-layout work, and
flight-recorder debugging of concurrency bugs, plus the commit and
merge-request conventions.

Merge branch 'ondrej/add-agents-skills' into 'main'

See merge request isc-projects/bind9!12363