chore: retire the stale future branch

[petercorke]

Summary

• `future` was 162 commits behind `main`, 0 ahead, last touched 2026-06-14 -- a leftover from before RTB used a proper PR-based workflow, no longer part of how anything actually merges.
• Confirmed via MVTB and bdsim (Peter's other actively-maintained repos): both already dropped their own `future` branches entirely. RTB was the outlier still carrying it.
• Third-party repos (`swift`, `spatialgeometry`) still use their own `future` branch as the live PR-merge target, set by their own owners -- unrelated, not touched here.

Changes:

• `ci.yml`: drop `future` from `push`/`pull_request` trigger branches
• `pull_request_template.md`: stop telling contributors to target `future`
• `README.md`: fix a stray image URL pointing at `future/docs/figs/...` -- would have 404'd once the branch is actually deleted

Test plan

• PR fix(teach): update of vellipse and fellipse #594 (the only open PR targeting `future`) retargeted to `main` separately, so it won't be orphaned
• After merge: delete the `future` branch on GitHub (holding off until explicitly confirmed, since it's a destructive remote action)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (1885394) to head (7039aca).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #605   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        142     142           
  Lines      13898   13898           
=====================================
  Misses     13898   13898           

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