ci: move every workflow to the org reusable templates

[eliotmcintire]

Supersedes #42. Every workflow becomes a thin caller on PredictiveEcology/actions
— 198 → 112 lines, and no workflow file here runs a job of its own any more.

Workflow	Before	After	Customisation
R-CMD-check	57	32	extra-config (matrix legs only)
test-coverage	36	29	none
pkgdown	48	19	none
revdeps	57	30	none

Three bugs this fixes

1. Neither R-CMD-check nor test-coverage has ever run on main. Both
triggered on [master, development]. There is no master branch, and the
default branch is main — so pushes and PRs to the default branch got no check
and no coverage at all.

2. The declared R floor was never tested. DESCRIPTION says R (>= 4.3).
With release at 4.6, that floor is oldrel-3, and the hand-rolled matrix
stopped at oldrel-2 — so the DESCRIPTION claim was unverified. Added.

3. revdeps ran on every pull_request, across three OSes. The shared
action's own README warns revdep checks are too resource intensive for standard
runners, and each revdep gets a full R CMD check with its dependency closure
installed first. That is hours of runner time per PR for a signal almost no PR
changes. Now workflow_dispatch plus the existing weekly schedule, on the
shared single-leg default.

It also drops the stale install-spatial-deps@v0.3 pin — what #43 was opened
to fix. Every tag of that action adds the ubuntugis PPA, whose libgdal37 is
ABI-incompatible with Posit's noble cache.

Customisations dropped, with reasons

You asked whether we need any. Assessed individually:

• OMP_THREAD_LIMIT: 4 — quickPlot has no src/ and no parallel or OpenMP
  use anywhere in R/. Inherited boilerplate.
• systemfonts in the revdeps extra-packages — not referenced in
  DESCRIPTION, R/, tests/ or vignettes/, and not transitively reachable
  from any declared dependency. Better restored if a revdep run actually needs
  it than carried on spec.
• The per-job [skip-ci] guards — now honoured by the shared workflows
  themselves (feat: honour [skip-ci] org-wide; add a reusable revdeps workflow actions#34), so callers no longer need them.

Everything else is the shared default.

⚠️ Behaviour changes

Coverage moves macOS → ubuntu and gains NOT_CRAN=true. skip_on_cran()
tests now run under coverage. That raises the measured figure — and can surface
tests that had never executed in CI. Require hit exactly that on this migration:
a test gated on NOT_CRAN and dpkg-query had never run anywhere, because
coverage was on macOS (no dpkg-query) and R CMD check skips skip_on_cran().
Worth watching this PR's coverage run for the same.

The matrix gains a nosuggests (_R_CHECK_DEPENDS_ONLY_) leg this repo
never had.

Why not #42

It is CONFLICTING, covers only three of the four workflows (leaving revdeps
hand-rolled), and carries unrelated DESCRIPTION, NEWS.md, README.md,
cran-comments.md and CRAN-SUBMISSION changes. Recommend closing it.

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