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/agent_provider.py

@@ -240,7 +240,7 @@ class AgentProvider(ABC):
         BottleBackend.provision_workspace against the running bottle."""
         from .log import info
 
-        manifest_bottle = plan.spec.manifest.bottle_for(plan.spec.agent_name)
+        manifest_bottle = plan.spec.manifest.bottle
         if manifest_bottle.git:
             from .git_gate import GIT_GATE_HOSTNAME, git_gate_render_gitconfig
             gate_host = getattr(plan, "git_gate_insteadof_host", GIT_GATE_HOSTNAME)