Move CLI facts out of the skill and into the CLI

The skill had grown into a reference manual: body type tables, auth
strategy lists, a template function table, field-by-field OAuth 2.0
config. All of it goes stale, because the user's CLI version and their
installed plugins decide what actually exists. On this machine a faker
plugin contributes 274 template functions; the skill listed eleven, and
got some names wrong.

The CLI should carry that knowledge, so:

- `yaak template-function list [filter]` and `template-function show
  <name>` report what the loaded plugins actually provide, the same way
  `request schema http` already merges in plugin auth strategies.
- `yaak folder schema` now exists, so folder payloads are discoverable
  like every other model. Required deriving JsonSchema on Folder.
- The request schema documents `bodyType` and `body` shapes, and states
  that setting a body type does not add a Content-Type header.

The skill drops to a single 135-line SKILL.md that teaches the model,
the workflows, and how to interrogate the CLI, and says outright that
the CLI wins when the two disagree.

crates/yaak-models/src/models.rs

@@ -949,7 +949,7 @@ pub struct ParentHeaders {
     pub headers: Vec<HttpRequestHeader>,
 }
 
-#[derive(Debug, Clone, PartialEq, Serialize, Deserialize, Default, TS)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize, Default, JsonSchema, TS)]
 #[serde(default, rename_all = "camelCase")]
 #[ts(export, export_to = "gen_models.ts")]
 #[enum_def(table_name = "folders")]