Mono-repo support

[ryancaille]

Seeing errors like:

ERROR: Template file not found at C:\my\repo.specify\templates\plan-template.md

and

ERROR: Template file not found at C:\my\repo.specify\templates\agent-file-template.md
INFO: Run specify init to scaffold .specify/templates, or add agent-file-template.md there.

This is because we have lots of projects in a single repo. All the files paths are assuming .specify is directly under Get-RepoRoot. but if we have installed spec-kit in a subfolder of the repo root, it has no way to find them.

The reason for installing spec-kit into the subfolder not the root is to have a constitution file per project.

if the scripts accepted an optional parameter that can be appended onto the repo root to locate the templates, this would solve this problem.

[aurexav]

#581