Adopt JiraPS v3 Issue type in JiraAgilePS issue cmdlets

[lipkau]

Problem

JiraAgilePS currently treats Agile issue payloads as JiraAgilePS-specific objects, while the underlying resource is the same Jira issue entity used by JiraPS. This creates avoidable model divergence and weaker cross-module pipeline interoperability.

Proposal

After JiraPS v3 is released, update JiraAgilePS issue-related cmdlets to use AtlassianPS.JiraPS.Issue as the canonical issue type for inputs/outputs where the resource is a Jira issue.

Acceptance criteria

• Get-JiraAgileIssue returns JiraPS issue objects (AtlassianPS.JiraPS.Issue) for all issue scopes.
• JiraAgilePS cmdlets that accept issue input support JiraPS issue typing/transformation.
• Existing board/backlog/sprint/epic issue retrieval behavior remains functionally equivalent.
• Unit tests cover JiraPS issue type interoperability in JiraAgilePS.
• User-facing docs and changelog are updated to describe the shared issue model.