Fix --no-skills flag not preventing skills from loading

The --no-skills flag set options.skills = [] in main.ts, but the
interactive mode UI would rediscover skills anyway because it called
loadSkills() directly instead of using the already-loaded skills.

Changes:
- Add AgentSession.skills and AgentSession.skillWarnings properties
- discoverSkills() now returns { skills, warnings } instead of Skill[]
- Interactive mode uses session.skills instead of calling loadSkills()
- Update SDK docs and examples for new return type

Fixes #577

packages/coding-agent/docs/sdk.md

@@ -528,7 +528,7 @@ eventBus.on("my-extension:status", (data) => console.log(data));
 import { createAgentSession, discoverSkills, type Skill } from "@mariozechner/pi-coding-agent";
 
 // Discover and filter
-const allSkills = discoverSkills();
+const { skills: allSkills, warnings } = discoverSkills();
 const filtered = allSkills.filter(s => s.name.includes("search"));
 
 // Custom skill
@@ -550,7 +550,7 @@ const { session } = await createAgentSession({
 });
 
 // Discovery with settings filter
-const skills = discoverSkills(process.cwd(), undefined, {
+const { skills } = discoverSkills(process.cwd(), undefined, {
   ignoredSkills: ["browser-*"],  // glob patterns to exclude
   includeSkills: ["search-*"],   // glob patterns to include (empty = all)
 });
@@ -747,7 +747,7 @@ const model = modelRegistry.find("provider", "id");   // Find specific model
 const builtIn = getModel("anthropic", "claude-opus-4-5"); // Built-in only
 
 // Skills
-const skills = discoverSkills(cwd, agentDir, skillsSettings);
+const { skills, warnings } = discoverSkills(cwd, agentDir, skillsSettings);
 
 // Hooks (async - loads TypeScript)
 // Pass eventBus to share pi.events across hooks/tools