Prepare spiceai plugin and automate marketplace distribution

[lukekim]

The plugin currently uses inconsistent client names, exposes a maintainer skill, and only uploads a ZIP on GitHub release publication. This change prepares spiceai for distribution through Claude, OpenAI, GitHub Copilot, and Grok.

• Standardize the plugin namespace as spiceai and use short skill names aligned with the CLI/API surfaces, such as $spiceai:sql and /spiceai:cloud. Add an SDK skill for the six official language clients.
• Remove improve-skills from public sources and exclude private maintenance files, eval tooling, credentials, and local caches from packages. The local maintainer copy remains outside version control.
• Make runtime installation a user-managed prerequisite in setup and cookbook guidance, and require an installed runtime before launch.
• Add Copilot marketplace metadata, bundled OpenAI listing assets, the declared MIT license, and catalogs that install from their own checkout.
• Add PR validation and reproducible ZIP/tar release builds with checksums, catalog entries pinned to the release commit, and submission instructions. Tag pushes create draft GitHub releases; publication and manual reruns update assets while preserving authored notes.

Validation:

• make check test-distribution release-preview — passed, including 13 distribution tests.
• actionlint, shellcheck for packaging/version scripts, ruff, and git diff --check — passed.
• Claude manifest/catalog validation and five setup response scenarios passed during preparation.

The namespace and skill renames require callers to update existing invocations. The plugin remains aligned with runtime v2.3.2. The existing published tag is unchanged; release automation applies to a future version/tag containing these changes and requires a clean checkout at that matching tag. Marketplace scans, portal publication, and external catalog PRs remain separate steps documented in docs/publishing.md. This does not rewrite historical commits or old release packages.