feat: add skill:// resource templates for Agent Skills discovery

Add MCP resource templates that expose Agent Skills (per agentskills.io
spec) from GitHub repositories via the skill:// URI scheme.

Resource templates:
- skill://{owner}/{repo}/{skill_name}/SKILL.md — fetch skill content
- skill://{owner}/{repo}/{skill_name}/_manifest — JSON manifest with
  repo:// URIs for each file, composing with existing repo:// resources

Discovery uses the Git Trees API to find SKILL.md files under known
paths (.github/skills/, skills/, .copilot/skills/).

Completions support for owner, repo (reusing existing resolvers), and
skill_name (discovers skills via tree search).

Registered under a new non-default 'skills' toolset. Enable with
--toolsets=skills or --toolsets=default,skills.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SamMorrowDrums

pkg/github/resources.go

@@ -15,5 +15,9 @@ func AllResources(t translations.TranslationHelperFunc) []inventory.ServerResour
 		GetRepositoryResourceCommitContent(t),
 		GetRepositoryResourceTagContent(t),
 		GetRepositoryResourcePrContent(t),
+
+		// Skill resources
+		GetSkillResourceContent(t),
+		GetSkillResourceManifest(t),
 	}
 }

pkg/github/server.go

@@ -200,6 +200,9 @@ func CompletionsHandler(getClient GetClientFn) func(ctx context.Context, req *mc
 			if strings.HasPrefix(req.Params.Ref.URI, "repo://") {
 				return RepositoryResourceCompletionHandler(getClient)(ctx, req)
 			}
+			if strings.HasPrefix(req.Params.Ref.URI, "skill://") {
+				return SkillResourceCompletionHandler(getClient)(ctx, req)
+			}
 			return nil, fmt.Errorf("unsupported resource URI: %s", req.Params.Ref.URI)
 		case "ref/prompt":
 			return nil, nil