Switch to post-merge air formatting on main

[lbm364dl]

Implements the post-merge-only option from #134. Rebased onto current main
(the branch predated ~630 merges) and wired to a token that can actually push.

What changes

• Remove the pre-merge format-suggest workflow.
• Add format-main — on push to main, install air, run air format .,
  and commit any changes back via git-auto-commit-action.
• Docs (CLAUDE.md, .github/CONTRIBUTING.md): five CI checks rather than
  six, and formatting documented as a local requirement with no pre-merge gate —
  run air format . yourself so the diff under review is the diff that lands
  and main does not accumulate formatting-only commits.
• The original commit also edited .github/copilot-instructions.md; that file
  has since been deleted from main, so the hunk is dropped.

Repo setting required before this works

main is protected by the "Not push in main" ruleset (pull_request rule,
1 approval). Its bypass list does not include github-actions, so the default
GITHUB_TOKEN cannot push. The job therefore checks out with
secrets.FORMAT_BOT_TOKEN.

That secret is a fine-grained PAT owned by a repository admin — repository
eduaguilera/whep, permission Contents: Read and write — and it is already
configured. Repository admins bypass the ruleset, so the ruleset itself needs no
change. The first job step fails with an explicit message when the secret is
missing, instead of dying on an opaque 403 at push time.

The commit carries [skip ci] so a formatting-only change does not re-run the
five-platform check suite. Unlike a GITHUB_TOKEN commit, a PAT-authored one
would trigger downstream workflows — drop [skip ci] if that is ever wanted.

Smoke test built into this PR

The second commit adds whitespace-only noise to R/validation_helpers.R, so
the first push to main gives format-main something to fix. Verified
locally: air 0.9.0 restores the file byte-for-byte, and lintr reports no lints,
because the deviations are confined to the four linters .lintr disables
precisely because air owns them. The auto-format commit on main should
therefore revert exactly that hunk and nothing else.

Note: format-suggest fails on this PR by design. It is a pull_request_target
workflow, so GitHub runs the copy from the base branch — it sees the deliberate
mis-format and objects. Deleting it is the point of this PR.

🤖 Generated with Claude Code

[github-actions]

[air] _{reported by reviewdog 🐶}

Suggested change

	.conservation_rel_error <- function(gridded,reference) {
	dplyr::case_when(
	.conservation_rel_error <- function(gridded, reference) {
	dplyr::case_when(

[github-actions]

[air] _{reported by reviewdog 🐶}

Suggested change

	gridded > 0 ~ Inf,
	.default = 0
	gridded > 0 ~ Inf,
	.default = 0