Add guest sessions and agent VM defaults

Add daemon-backed workspace and guest-session primitives so host
orchestrators can prepare /root/repo, launch long-lived guest commands,
and attach to pipe-mode sessions over the local stdio mux bridge.

Persist richer session metadata and launch diagnostics, preflight guest
cwd/command requirements, make pipe-mode attach rehydratable from guest
state after daemon restart, and allow submodules when workspace prepare
runs in full_copy mode.

At the same time, stop vm run from auto-attaching opencode, make it
print next-step commands instead, and make glibc guest images more
agent-ready by installing node, opencode, claude, and pi while syncing
opencode/claude/pi auth files into work disks on VM start.

Validation:
- GOCACHE=/tmp/banger-gocache go test ./...
- make build
- banger vm workspace prepare --help
- banger vm session --help
- banger vm session start --help
- banger vm session attach --help

internal/daemon/capabilities.go

@@ -210,7 +210,13 @@ func (workDiskCapability) PrepareHost(ctx context.Context, d *Daemon, vm *model.
 	if err := d.ensureGitIdentityOnWorkDisk(ctx, vm); err != nil {
 		return err
 	}
-	return d.ensureOpencodeAuthOnWorkDisk(ctx, vm)
+	if err := d.ensureOpencodeAuthOnWorkDisk(ctx, vm); err != nil {
+		return err
+	}
+	if err := d.ensureClaudeAuthOnWorkDisk(ctx, vm); err != nil {
+		return err
+	}
+	return d.ensurePiAuthOnWorkDisk(ctx, vm)
 }
 
 func (workDiskCapability) AddDoctorChecks(_ context.Context, d *Daemon, report *system.Report) {