chore(skills): add reset-caches skill; close caller-identity decision

reset-caches does the whole-box reset (kill servers, test binaries and
test containers, clear Go/lint/Docker caches and stale scratch). Scratch
root follows TMPDIR, not GOTMPDIR, since GOTMPDIR now lives on /mnt/nvme
while Claude scratch stays under TMPDIR.

Closes gopherstack-cu4g: Option A is built (awsmeta.Principal wired via
principalMiddleware). Unconverted consumers filed as gopherstack-tnqy.6.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: Witness Patrol