Skills part 4: editing skills in Settings, with the built-ins as examples

[shanforge]

Part of #56, and the last box of #64 — which closes it.

Scope

• Editable in Settings, with the built-ins readable as examples

What this is for

The file format from the first skills issue makes a skill readable in any editor. This
makes it editable without leaving the app, and — the part that is easy to skip — makes the
built-ins readable as examples, so someone writing their first skill has a worked one
in front of them rather than an empty box.

• A built-in is readable, and editing one does not destroy it. Whatever the edit model
  is (copy-on-write, revert, or refusing the edit), it is decided deliberately and stated,
  because a user who has overwritten the only example cannot get it back.
• Validation is the store's, not the form's. The name rules and bounds already exist
  from the first issue; the editor reports them rather than re-implementing them, or the
  two will disagree.
• Export and import are reachable from here, since this is where someone looks for them.
• Tool scoping is edited as a narrowing, and the UI says so — a tool the user has
  disabled globally does not become available by being named in a skill.

Ground rules

Same as #56. This closes #64, so it also carries the click-through evidence for the whole
skills feature: the screenshots #56's ground rules ask for.