fix: keep global install dry-run out of user state

[Aryan Singh K. (aryansk)]

Summary

Make apm install --dry-run -g truly read-only when bootstrapping a user-scope manifest.

Changes

• skip ensure_user_dirs() during dry-run
• redirect an absent global apm.yml to temporary storage so validation and planning can still parse a manifest
• keep user-facing output pointed at the real global manifest path
• clean up temporary preview state in the command finally path
• add regressions proving an absent user manifest does not create ~/.apm

Validation

Prepared focused regression tests:
pytest -q tests/unit/regressions/test_issue_2549_global_dry_run.py

Patch structure was validated with git apply --check. Full repository tests should run in CI.

Fixes #2549