refactor(manifest): split Manifest into ManifestIndex + Manifest single-value type

Manifest now holds exactly one agent and one effective bottle (with
git_user overlay already applied). The old multi-agent/bottle
collection is renamed ManifestIndex. BottleSpec.manifest starts as
ManifestIndex from the CLI and becomes Manifest after _validate()
calls load_for_agent(); all provisioning code downstream reads
spec.manifest.agent / spec.manifest.bottle instead of indexing by name.

bot_bottle/contrib/pi/agent_provider.py

@@ -232,7 +232,7 @@ class PiAgentProvider(AgentProvider):
     def provision_skills(self, plan: "BottlePlan", bottle: "Bottle") -> None:
         from ...backend.util import host_skill_dir
 
-        agent = plan.spec.manifest.agents[plan.spec.agent_name]
+        agent = plan.spec.manifest.agent
         if not agent.skills:
             return
         skills_dir = _skills_dir(plan.guest_home)