Give bots skills: runbooks they load when a job matches

A skill is a runbook: a name, a unique slug, a one-line description of when
it applies, and the markdown body. A run only ever sees the slug and
description (one load_skill tool carries them), and reads the whole body
only when it calls the tool, so a long runbook costs nothing until needed.

Skills are the third extension kind beside MCP servers and custom tools, on
the same botExtensionsSql path, so per-bot, per-workspace and all-bots
grants work the same way, and attachDefaultExtensions hands an every-bot
skill to new bots. load_skill is a reserved tool name, so a custom tool
cannot shadow it, and a built-in wins a clash.

Managed on Settings > Skills; ExtensionRow links each kind to its own page
and a skill never reports a failure, since nothing connects or runs.

src/hooks/use-extensions.ts

@@ -18,7 +18,7 @@ type Params = {
 type Status = { error?: boolean; success?: boolean; msg?: string };
 
 /**
- * A bot's or workspace's MCP servers and tools, with adding and removing
+ * A bot's or workspace's MCP servers, tools and skills, with adding and removing
  * them (each asks first). Shared by the bot and workspace settings pages
  * and the chat's composer.
  */