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/examples/sdk/04-skills.ts

@@ -8,11 +8,14 @@
 import { createAgentSession, discoverSkills, SessionManager, type Skill } from "@mariozechner/pi-coding-agent";
 
 // Discover all skills from cwd/.pi/skills, ~/.pi/agent/skills, etc.
-const allSkills = discoverSkills();
+const { skills: allSkills, warnings } = discoverSkills();
 console.log(
 	"Discovered skills:",
 	allSkills.map((s) => s.name),
 );
+if (warnings.length > 0) {
+	console.log("Warnings:", warnings);
+}
 
 // Filter to specific skills
 const filteredSkills = allSkills.filter((s) => s.name.includes("browser") || s.name.includes("search"));