feat: release pipeline for prebuilt plugin assets #26

Part of the C++ plugin migration (upstream: WasmEdge/WasmEdge#3840). Depends on the pilot (see Relationships) — needs at least one building plugin to exercise the pipeline.

Plugins must ship as prebuilt release assets from this repo so the installer (and users) stop depending on WasmEdge core releases.

• Decide the tag/versioning scheme. Proposal: repo-wide tags <major>.<minor>.<patch>, with each release documenting which WasmEdge release the plugins were built against (e.g. "built against WasmEdge 0.17.0").
• Asset naming: keep it compatible with what the installer already parses for plugin assets in core releases — WasmEdge-plugin-<plugin_name>-<version>-<os>_<arch>.tar.gz — so the installer change stays a URL swap.
• release.yml workflow: on tag push, build every enabled plugin across the platform matrix and upload assets to the GitHub release.
• Include per-asset SHA256 checksums in the release.
• Document the release process (who tags, when — at minimum after each WasmEdge release bump) in docs/RELEASE.md.
• Acceptance: a 0.1.0 pre-release exists containing at least the zlib plugin asset for one platform, installable by hand.

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