fix: add help output for supabase stdio CLI

[mojobeeping]

Summary

• add --help and -h output for the Supabase stdio CLI
• convert parseArgs failures into a concise error plus usage text with exit code 1
• add stdio integration coverage for help and unknown flags

Closes #296.

Testing

• npx -y pnpm@10 build
• CI=true npx -y pnpm@10 --filter @supabase/mcp-server-supabase test:integration -- test/stdio.integration.ts --run
• npx -y pnpm@10 exec biome check packages/mcp-server-supabase/src/transports/stdio.ts packages/mcp-server-supabase/test/stdio.integration.ts
• node packages/mcp-server-supabase/dist/transports/stdio.js --help
• node packages/mcp-server-supabase/dist/transports/stdio.js --unknown-flag

[mojobeeping]

Closing this as a duplicate of #297, which was opened before this PR while I was preparing the same fix.