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/data/app-data.ts

@@ -283,6 +283,14 @@ export const AppData = {
      */
     CustomToolDefaultTimeoutMs: 60 * 1000,
     CustomToolMaxTimeoutMs: 10 * 60 * 1000,
+    /**
+     * Skills (Settings, under Skills): how long one's body may be, and how
+     * much of it `load_skill` hands back in one call. A runbook past that
+     * is worth splitting into two skills.
+     */
+    SkillBodyMaxChars: 60000,
+    SkillBodyShownChars: 30000,
+    SkillDescriptionMaxChars: 300,
     /**
      * `search_computers`: how long the command may run on each computer,
      * and how much of each computer's output the bot reads.