This PR does a few improvements for the prompts library in Cody Web

• First of all, it fixes recent regression in openctx (apparently internally openctx library tries to find vs code setting file and with an empty Cody Web Workspace URI it fails in runtime and therefore doesn't produce any openctx providers such as remote files or repositories)
• It fixes prompts editor placeholder appearance (we no longer add whitespaces to the field unconditionally)
• Disables non-runnable in Cody Web prompts (insert or inline commands) before we render them and these actions did nothing

Test plan

• Check that you can use openctx mentions in Cody Web (with Cody Web linked in sg, check ./web contribution doc to see how to link Cody Web and sg instance)
• Check that you can't click non-chat prompts in Cody Web and see that they are disabled
• Check that you can see the prompts library placeholder if you don't have context (Cody Web standalone page case)

Changelog