feat(actions): add goto_file_edit_close to navigate and dispose view

[dlyongemallo]

Closes #135.

[Copilot]

Pull request overview

Adds a new public action to open the inferred file in the previous non-diffview tabpage (same routing as goto_file_edit) and then close/dispose the originating diffview, addressing the workflow requested in #135.

Changes:

• Add actions.goto_file_edit_close() to navigate to the inferred file and then close + dispose the current view.
• Refactor internal open_goto_file() helper to return a boolean success value for callers that need to react to “no inferred file”.
• Extend functional tests and help docs to cover the new action.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
lua/diffview/actions.lua	Introduces goto_file_edit_close and adds an ok return from open_goto_file() so the new action can skip closing when no file is inferred.
lua/diffview/tests/functional/actions_spec.lua	Adds coverage for export presence, error behavior without an active view, command routing, and ensuring the view is closed/disposed only after successful navigation.
doc/diffview.txt	Documents the new action and lists it among unused actions for discoverability.

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