test: delete dead Pi key-files E2E (feature removed in Dreamer V2)

pi-key-files-v6.test.ts asserts Pi's system-prompt injector renders a
<key-files> block, but key-files injection was fully removed this release
(the feature moved to AFT's dreamer; the plugin-side key-files-v6.test.ts was
already deleted, this Pi sibling was missed). The host Pi E2E correctly failed
on 'Expected to contain: <key-files>'. No live <key-files> injection remains;
pi-cache-stability.test.ts only references it in a comment + a skipped test.
CI globs tests/pi-*.test.ts so no workflow change is needed.

Co-authored-by: Alfonso [Magic Context] <288211368+alfonso-magic-context@users.noreply.github.com>

Author: ualtinok