Feature: Open result action for FileOut and figure/report outputs #65

Goal

Make outputs such as Guess the Condition's PDF report accessible directly from Fiji, Napari and QuPath. The existing FileOut gives a path but not a portable request to open it.

This was already explored in Wave 2 specification PR #39, which proposes FigureOut, Open() and LinkOut. Reuse that work after reviewing its security policy; avoid creating overlapping result types for the same use case.

Proposed MVP

• Each host shows a discoverable Open result action for an existing FileOut whose path is still accessible, ideally alongside a Show in folder action.
• Preview formats such as PNG, PDF or HTML only when supported and safe; don't assume every host can render them.
• Optional Open() hint can request post-run opening, but execution is host-authorized (never execute file contents or untrusted URLs by default).
• Preserve generated files before cleanup; address Napari #32 as a prerequisite.

Acceptance criteria

• PDF generated by Guess the Condition remains on disk and is directly openable after the run.
• An invalid, missing or untrusted file type is presented clearly, without launching arbitrary executables.
• Supported file/preview behavior and safe fallback are documented across hosts.
• CLI/notebooks provide stable output paths without GUI-only side effects.

Priority: Medium; small presentation capability first, general FigureOut later.