test(workbench): add RStudio Git-pane commit/push scenario (validated against live Workbench)

Context

PR #363 (Git operations from Workbench sessions, closes #306) originally included a "Commit and push using the RStudio Git pane" scenario plus a RStudioGitPane page object. It was dropped before merge because:

1. The selectors in RStudioGitPane were never validated against a live Workbench instance (the code carried a TODO note to that effect).
2. The working-directory setup used Rscript -e 'setwd(...)' run in the terminal, which spawns a separate R process and does not change the already-running RStudio session's working directory — so the Git pane would never discover the cloned repo.

The terminal-based Git scenarios (RStudio/VSCode/Positron clone + branch/commit/push) shipped in #363 and work.

What's needed

• Re-add the RStudio Git-pane GUI scenario, with selectors verified against a live Workbench (e.g. ganso01-staging).
• Set the session working directory via rstudio_eval(page, 'setwd(...)') (in-session), not a terminal Rscript subprocess.
• Re-introduce a validated RStudioGitPane page object.

References

• PR #363
• Original feature issue #306

Start with the terminal-based Git scenarios from PR #363, the dropped RStudioGitPane page object, and the rstudio_eval entry point. Validate the RStudio Git-pane selectors against the referenced live Workbench, set the working directory in-session, and finish when the GUI commit/push scenario runs successfully.