feat(skills): adopt skill://github/<name>/SKILL.md URI prefix

Per SEP-2640, the bundled-skill URI structure is
skill://<skill-path>/<file-path>, where preceding segments before the
final name are an organizational prefix chosen by the server. This
commit moves from the flat skill://<name>/SKILL.md form to the
prefixed skill://github/<name>/SKILL.md form, matching the convention
used in github#2374.

Single-line change to skills.Bundled.URI(). Tests already build URIs
through this helper, so they pick up the change automatically — no
test edits needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: olaservo

skills/registry.go

@@ -40,8 +40,10 @@ type Bundled struct {
 	Enabled func() bool
 }
 
-// URI returns the skill's canonical SKILL.md URI: skill://<name>/SKILL.md.
-func (b Bundled) URI() string { return "skill://" + b.Name + "/SKILL.md" }
+// URI returns the skill's canonical SKILL.md URI: skill://github/<name>/SKILL.md.
+// The "github/" segment is the SEP's organizational prefix for this server's
+// bundled skills; the final path segment is the skill name.
+func (b Bundled) URI() string { return "skill://github/" + b.Name + "/SKILL.md" }
 
 func (b Bundled) enabled() bool { return b.Enabled == nil || b.Enabled() }