docs: import guidelines for future re-imports + PR #44 leftovers

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Part of the C++ plugin migration (upstream: WasmEdge/WasmEdge#3840). Follow-up to the source import (#23, landed via PR #44).

Write an import guidelines doc (e.g. docs/IMPORTING.md) so future refreshes/re-imports from WasmEdge core are mechanical, and close out the leftovers from PR #44.

Guidelines doc

• Document the import recipe: fresh upstream clone → git filter-repo over the path list (record the exact list used in PR feat: import plugin sources, tests, and CI utils from WasmEdge core #44: plugins, test/plugins, thirdparty/wasi_crypto, utils/docker, utils/ffmpeg, utils/opencvmini, utils/wasi-crypto, utils/wasi-nn, utils/wasi-test, utils/build_libpiper.sh) → git rebase --onto main --root --empty=drop --committer-date-is-author-date → push branch + open PR
• Document verification steps: HEAD tree = main's files + imported paths only; git rev-list --count and 0 merge commits (linear); git blame spot checks for author continuity
• Document known check behavior on import PRs: DCO flags upstream squash-merge artifacts (author rewritten to GitHub noreply while the sign-off keeps the real email) — resolve via the "Set DCO to pass" button on the PR Checks tab (the check's "Details" link goes to the probot app page, which has no button); clang-format flags generated/fixture paths
• Do not document merge mechanics that involve lifting main's protection (maintainer decision, 2026-06-12) — that is acceptable only while this repo is WIP and lives in the section below, not in the doc. The doc may point at this issue for the migration-window procedure.

Migration-window sync procedure (kept here on purpose, not in the doc)

For syncing from WasmEdge core before this repo is ready. Expires when the migration completes (#28): after that, main's protection is never removed again.

1. Re-run the import recipe above against current upstream master → push branch, open PR for review
2. Merge by fast-forward to preserve the reviewed SHAs: admin temporarily lifts ruleset 2835007 → git push origin <branch>:main → re-add the ruleset. GitHub auto-marks the PR merged and closes linked issues
3. Alternative that leaves protection untouched: the "Rebase and merge" button (allowed by the ruleset) — but it rewrites all SHAs and committer info, breaking review references

Reference run: PR #44 (resolution summary).

Leftovers from PR #44

• Lint config follow-up: add FILTER_REGEX_EXCLUDE for thirdparty/.* and test/plugins/wasm_bpf/assets/.* to .github/workflows/super-linter.yml
• (optional, for the record) retroactively click "Set DCO to pass" on the merged PR's check run — done 2026-06-12, DCO check on 18ce2cd is green

Already tracked elsewhere, listed for completeness: keep test/plugins/wasi_logging disabled until its target exists → #24; reformat test/plugins/wasi_crypto/asymmetric.cpp under the pinned clang-format → #24; fix [[ ! -v "${VAR}" ]] checks in utils/wasi-nn scripts → #31.

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