fix: MCP registry publication in release workflow

[mattrossman]

• Specifies per-platform asset_pattern for MCP Publisher in mise.toml so mise picks the correct release asset (mcp-publisher_* instead of registry_*)
• Add MCP registry login step before publish using domain HTTP verification (our listing is under com.supabase, which requires the DOMAIN_VERIFICATION_KEY secret — see packages/mcp-server-supabase/scripts/registry/login.sh)
• Add workflow_dispatch trigger so the workflow can be manually re-run without a dummy commit (e.g. to recreate a closed release PR)
• Update contributing guide with a Releases section explaining the automated flow, and reframe manual MCP registry publish as an optional fallback, see https://github.com/supabase-community/supabase-mcp/blob/mattrossman/ai-695-fix-mcp-registry-publication-in-release-workflow/CONTRIBUTING.md#releases

I've populated DOMAIN_VERIFICATION_KEY in repo secrets.

Verification

I created (and reverted) a temp test workflow that installs mcp-publisher and runs mcp-publisher --version, see that successful run here.

https://github.com/supabase-community/supabase-mcp/actions/runs/25227199423/job/73973577890?pr=268

References:

• https://modelcontextprotocol.io/registry/github-actions

Closes AI-695