Isolate smoke-test dry-run state

[SS-42]

Summary

This isolates smoke-test dry-run subcommands from the developer's real home directory and cache.

Phase 6 now runs install --dry-run, uninstall --dry-run, update --dry-run, and the config set/get/reset round trip under a temporary HOME, XDG_CONFIG_HOME, APPDATA, LOCALAPPDATA, CBM_CACHE_DIR, and private PATH. The temporary tree is removed by the existing cleanup trap.

Observed environment: macOS 26.5.2 (25F84), Darwin 25.5.0 arm64, Apple clang 21.0.0. The isolation is shell/environment based and should apply across supported platforms.

Test plan

• make -f Makefile.cbm build/c/codebase-memory-mcp CC=clang CXX=clang++
• scripts/smoke-test.sh build/c/codebase-memory-mcp

[DeusData]

Thanks for tightening the smoke-test dry-run isolation. Triage: UX/test-infra safety change touching scripts.

Because this changes script behavior around HOME, XDG, APPDATA, PATH, and cache directories, review will check that dry-run commands cannot mutate a developer real config and that cleanup is reliable on failure. Please keep the test evidence in the PR description current after any updates.

[DeusData]

Thanks — this fixes a real hygiene bug (the smoke battery was mutating the developer's actual config via 'config set'). Env-only isolation into a script-owned mktemp home, byte-identical assertions, clean teardown. One non-blocking question we'll keep an eye on: whether Windows resolves the profile via USERPROFILE/known-folder APIs in any of these paths — worst case isolation is incomplete there, never worse than today. Merging.