Centralize provisioning on macOS 26

[hisaac]

Summary

• Move the existing macOS 26 Xcode and simulator preset into the shared agent configuration.
• Remove OS-specific presets and dynamic version-based configuration loading.
• Restrict Ansible macOS updates to 26.x and remove legacy Gatekeeper handling and obsolete Sonoma commentary.
• Update the README and task descriptions to document macOS 26 as the only supported guest OS.

Stack

Depends on #17. macOS 27 support is intentionally deferred to a subsequent PR.

Validation

• Ansible lint passed, including the pre-commit checks.
• TOML validation and git diff whitespace checks passed.
• Ansible inventory loading verified the macOS 26 Xcode/runtime/simulator preset; playbook syntax check passed.
• Live VM provisioning has not been run.