test(memory): isolate the global config dir per test file (CI fix) - #237

Second dev-CI failure investigation (run 31648866555) with the
identity-fixture fix in place: the same assertion still failed, still
with no visible warning. Verified root cause: bun test runs all files in
ONE process, sequentially, sharing one XDG_CONFIG_HOME. A file that runs
before this one and triggers global-memory initialization (memory.test
prepare, or instance boot via bootstrap.ts memory.init) leaves a VALID
global memory.jsonc whose model this file's fake provider does not know.
writeGlobal then silently no-ops over the valid file (config.ts returns
false), configuration() loads the foreign model, resolveModel fails (the
warning is captured by TestConsole and never reaches CI logs), and
search fails closed with "unavailable". R1-00's identical flow passes
because R1-07's finalizer removes the global file in between. CI-only
because bun's file order is deterministic per filesystem state and the
fresh CI checkout orders a contaminator before this file.

Fix: pin a private OPENCODE_CONFIG_DIR per file (beforeAll/afterAll,
live env getter — globalConfigDir reads it at call time) so the global
file can never be contaminated by earlier files, plus a tripwire
assertion after writeGlobal that the loaded global config carries the
expected model.

Registered separately (product hardening): writeGlobal should log when
it declines to overwrite an existing valid config.

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