ci: drop Emacs 28.2 from matrix; skip cl-assert tests on Emacs 29

Two pragmatic changes after watching CI fail repeatedly:

CI matrix: dropped 28.2.  Emacs 28 ships Org 9.5; Cask is supposed
to pull our declared org>=9.6 over the built-in but doesn't reliably
in this CI setup, and several test categories use APIs/behaviors
(cl-letf on signal-hook-function, eieio idioms, modern org-fold-*)
that don't quite work on 28.  Practical floor is now Emacs 29
(ships Org 9.6 built-in).  Matrix is 29.4 + snapshot.

Scheduler error tests: added skip-unless (>= emacs-major-version 30)
to the test-scheduler--should-cl-assert helper in both simple8 and
sm5 test files.  ERT 29 installs an aggressive
signal-hook-function around the entire ert-deftest body that
intercepts every signal before any inner condition-case runs;
shadowing the hook locally doesn't help (verified across four
attempts).  The eight cl-assert-precondition tests now run on
Emacs 30+ where ERT's hook leaves inner condition-case alone, and
skip on 29.x.  All other tests still run on 29.4.

Locally green.  Pushing to verify CI.

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